diff --git a/packages/moxxmpp/lib/src/managers/namespaces.dart b/packages/moxxmpp/lib/src/managers/namespaces.dart
index 03e40e0..713493d 100644
--- a/packages/moxxmpp/lib/src/managers/namespaces.dart
+++ b/packages/moxxmpp/lib/src/managers/namespaces.dart
@@ -31,3 +31,4 @@ const lastMessageCorrectionManager = 'org.moxxmpp.lastmessagecorrectionmanager';
 const messageReactionsManager = 'org.moxxmpp.messagereactionsmanager';
 const stickersManager = 'org.moxxmpp.stickersmanager';
 const entityCapabilitiesManager = 'org.moxxmpp.entitycapabilities';
+const messageProcessingHintManager = 'org.moxxmpp.messageprocessinghint';
diff --git a/packages/moxxmpp/lib/src/message.dart b/packages/moxxmpp/lib/src/message.dart
index 1995cb5..701d8ce 100644
--- a/packages/moxxmpp/lib/src/message.dart
+++ b/packages/moxxmpp/lib/src/message.dart
@@ -40,15 +40,6 @@ class MessageBodyData {
       text: body,
     );
   }
-
-  static List<XMLNode> messageSendingCallback(TypedMap extensions) {
-    if (extensions.get<ReplyData>() != null) {
-      return [];
-    }
-
-    final data = extensions.get<MessageBodyData>();
-    return data != null ? [data.toXML()] : [];
-  }
 }
 
 class MessageIdData {
@@ -113,11 +104,16 @@ class MessageDetails {
 }
 
 class MessageManager extends XmppManagerBase {
-  MessageManager(this.messageSendingCallbacks) : super(messageManager);
+  MessageManager() : super(messageManager);
 
   /// A list of callbacks that are called when a message is sent in order to add
   /// appropriate child elements.
-  final List<MessageSendingCallback> messageSendingCallbacks;
+  final List<MessageSendingCallback> _messageSendingCallbacks =
+      List<MessageSendingCallback>.empty(growable: true);
+
+  void registerMessageSendingCallback(MessageSendingCallback callback) {
+    _messageSendingCallbacks.add(callback);
+  }
 
   @override
   List<StanzaHandler> getIncomingStanzaHandlers() => [
@@ -190,7 +186,7 @@ class MessageManager extends XmppManagerBase {
           to: to.toString(),
           id: extensions.get<MessageIdData>()?.id,
           type: 'chat',
-          children: messageSendingCallbacks
+          children: _messageSendingCallbacks
               .map((c) => c(extensions))
               .flattened
               .toList(),
@@ -387,4 +383,30 @@ class MessageManager extends XmppManagerBase {
       ),
     );
   }
+
+  List<XMLNode> _messageSendingCallback(TypedMap extensions) {
+    if (extensions.get<ReplyData>() != null) {
+      return [];
+    }
+    if (extensions.get<StickersData>() != null) {
+      return [];
+    }
+    if (extensions.get<StatelessFileSharingData>() != null) {
+      return [];
+    }
+    if (extensions.get<OOBData>() != null) {
+      return [];
+    }
+
+    final data = extensions.get<MessageBodyData>();
+    return data != null ? [data.toXML()] : [];
+  }
+
+  @override
+  Future<void> postRegisterCallback() async {
+    await super.postRegisterCallback();
+
+    // Register the sending callback
+    registerMessageSendingCallback(_messageSendingCallback);
+  }
 }
diff --git a/packages/moxxmpp/lib/src/namespaces.dart b/packages/moxxmpp/lib/src/namespaces.dart
index f0be542..eb98e63 100644
--- a/packages/moxxmpp/lib/src/namespaces.dart
+++ b/packages/moxxmpp/lib/src/namespaces.dart
@@ -96,7 +96,7 @@ const httpFileUploadXmlns = 'urn:xmpp:http:upload:0';
 // XEP-0372
 const referenceXmlns = 'urn:xmpp:reference:0';
 
-// XEP-380
+// XEP-0380
 const emeXmlns = 'urn:xmpp:eme:0';
 const emeOtr = 'urn:xmpp:otr:0';
 const emeLegacyOpenPGP = 'jabber:x:encrypted';
diff --git a/packages/moxxmpp/lib/src/xeps/staging/file_upload_notification.dart b/packages/moxxmpp/lib/src/xeps/staging/file_upload_notification.dart
index 51d60ff..c429cc8 100644
--- a/packages/moxxmpp/lib/src/xeps/staging/file_upload_notification.dart
+++ b/packages/moxxmpp/lib/src/xeps/staging/file_upload_notification.dart
@@ -2,8 +2,11 @@ import 'package:moxxmpp/src/managers/base.dart';
 import 'package:moxxmpp/src/managers/data.dart';
 import 'package:moxxmpp/src/managers/handlers.dart';
 import 'package:moxxmpp/src/managers/namespaces.dart';
+import 'package:moxxmpp/src/message.dart';
 import 'package:moxxmpp/src/namespaces.dart';
 import 'package:moxxmpp/src/stanza.dart';
+import 'package:moxxmpp/src/stringxml.dart';
+import 'package:moxxmpp/src/util/typed_map.dart';
 import 'package:moxxmpp/src/xeps/xep_0446.dart';
 
 /// NOTE: Specified by https://github.com/PapaTutuWawa/custom-xeps/blob/master/xep-xxxx-file-upload-notifications.md
@@ -15,6 +18,16 @@ class FileUploadNotificationData {
 
   /// The file metadata indicated in the upload notification.
   final FileMetadataData metadata;
+
+  XMLNode toXML() {
+    return XMLNode.xmlns(
+      tag: 'file-upload',
+      xmlns: fileUploadNotificationXmlns,
+      children: [
+        metadata.toXML(),
+      ],
+    );
+  }
 }
 
 /// Indicates that a file upload has been cancelled.
@@ -23,6 +36,16 @@ class FileUploadNotificationCancellationData {
 
   /// The id of the upload notifiaction that is cancelled.
   final String id;
+
+  XMLNode toXML() {
+    return XMLNode.xmlns(
+      tag: 'cancelled',
+      xmlns: fileUploadNotificationXmlns,
+      attributes: {
+        'id': id,
+      },
+    );
+  }
 }
 
 /// Indicates that a file upload has been completed.
@@ -31,6 +54,16 @@ class FileUploadNotificationReplacementData {
 
   /// The id of the upload notifiaction that is replaced.
   final String id;
+
+  XMLNode toXML() {
+    return XMLNode.xmlns(
+      tag: 'replaces',
+      xmlns: fileUploadNotificationXmlns,
+      attributes: {
+        'id': id,
+      },
+    );
+  }
 }
 
 class FileUploadNotificationManager extends XmppManagerBase {
@@ -107,4 +140,33 @@ class FileUploadNotificationManager extends XmppManagerBase {
         ),
       );
   }
+
+  List<XMLNode> _messageSendingCallback(TypedMap extensions) {
+    final fun = extensions.get<FileUploadNotificationData>();
+    if (fun != null) {
+      return [fun.toXML()];
+    }
+
+    final cancel = extensions.get<FileUploadNotificationCancellationData>();
+    if (cancel != null) {
+      return [cancel.toXML()];
+    }
+
+    final replace = extensions.get<FileUploadNotificationReplacementData>();
+    if (replace != null) {
+      return [replace.toXML()];
+    }
+
+    return [];
+  }
+
+  @override
+  Future<void> postRegisterCallback() async {
+    await super.postRegisterCallback();
+
+    // Register the sending callback
+    getAttributes()
+        .getManagerById<MessageManager>(messageManager)
+        ?.registerMessageSendingCallback(_messageSendingCallback);
+  }
 }
diff --git a/packages/moxxmpp/lib/src/xeps/xep_0066.dart b/packages/moxxmpp/lib/src/xeps/xep_0066.dart
index a008be3..5b0d51f 100644
--- a/packages/moxxmpp/lib/src/xeps/xep_0066.dart
+++ b/packages/moxxmpp/lib/src/xeps/xep_0066.dart
@@ -2,9 +2,11 @@ import 'package:moxxmpp/src/managers/base.dart';
 import 'package:moxxmpp/src/managers/data.dart';
 import 'package:moxxmpp/src/managers/handlers.dart';
 import 'package:moxxmpp/src/managers/namespaces.dart';
+import 'package:moxxmpp/src/message.dart';
 import 'package:moxxmpp/src/namespaces.dart';
 import 'package:moxxmpp/src/stanza.dart';
 import 'package:moxxmpp/src/stringxml.dart';
+import 'package:moxxmpp/src/util/typed_map.dart';
 
 /// A data class representing the jabber:x:oob tag.
 class OOBData {
@@ -65,4 +67,23 @@ class OOBManager extends XmppManagerBase {
         ),
       );
   }
+
+  List<XMLNode> _messageSendingCallback(TypedMap extensions) {
+    final data = extensions.get<OOBData>();
+    return data != null
+        ? [
+            data.toXML(),
+          ]
+        : [];
+  }
+
+  @override
+  Future<void> postRegisterCallback() async {
+    await super.postRegisterCallback();
+
+    // Register the sending callback
+    getAttributes()
+        .getManagerById<MessageManager>(messageManager)
+        ?.registerMessageSendingCallback(_messageSendingCallback);
+  }
 }
diff --git a/packages/moxxmpp/lib/src/xeps/xep_0085.dart b/packages/moxxmpp/lib/src/xeps/xep_0085.dart
index 62c788e..1074696 100644
--- a/packages/moxxmpp/lib/src/xeps/xep_0085.dart
+++ b/packages/moxxmpp/lib/src/xeps/xep_0085.dart
@@ -2,6 +2,7 @@ import 'package:moxxmpp/src/managers/base.dart';
 import 'package:moxxmpp/src/managers/data.dart';
 import 'package:moxxmpp/src/managers/handlers.dart';
 import 'package:moxxmpp/src/managers/namespaces.dart';
+import 'package:moxxmpp/src/message.dart';
 import 'package:moxxmpp/src/namespaces.dart';
 import 'package:moxxmpp/src/stanza.dart';
 import 'package:moxxmpp/src/stringxml.dart';
@@ -53,11 +54,6 @@ enum ChatState {
       xmlns: chatStateXmlns,
     );
   }
-
-  static List<XMLNode> messageSendingCallback(TypedMap extensions) {
-    final data = extensions.get<ChatState>();
-    return data != null ? [data.toXML()] : [];
-  }
 }
 
 class ChatStateManager extends XmppManagerBase {
@@ -109,4 +105,23 @@ class ChatStateManager extends XmppManagerBase {
       ),
     );
   }
+
+  List<XMLNode> _messageSendingCallback(TypedMap extensions) {
+    final data = extensions.get<ChatState>();
+    return data != null
+        ? [
+            data.toXML(),
+          ]
+        : [];
+  }
+
+  @override
+  Future<void> postRegisterCallback() async {
+    await super.postRegisterCallback();
+
+    // Register the sending callback
+    getAttributes()
+        .getManagerById<MessageManager>(messageManager)
+        ?.registerMessageSendingCallback(_messageSendingCallback);
+  }
 }
diff --git a/packages/moxxmpp/lib/src/xeps/xep_0184.dart b/packages/moxxmpp/lib/src/xeps/xep_0184.dart
index aa8e5a5..76aa678 100644
--- a/packages/moxxmpp/lib/src/xeps/xep_0184.dart
+++ b/packages/moxxmpp/lib/src/xeps/xep_0184.dart
@@ -4,6 +4,7 @@ import 'package:moxxmpp/src/managers/base.dart';
 import 'package:moxxmpp/src/managers/data.dart';
 import 'package:moxxmpp/src/managers/handlers.dart';
 import 'package:moxxmpp/src/managers/namespaces.dart';
+import 'package:moxxmpp/src/message.dart';
 import 'package:moxxmpp/src/namespaces.dart';
 import 'package:moxxmpp/src/stanza.dart';
 import 'package:moxxmpp/src/stringxml.dart';
@@ -25,15 +26,6 @@ class MessageDeliveryReceiptData {
       xmlns: deliveryXmlns,
     );
   }
-
-  static List<XMLNode> messageSendingCallback(TypedMap extensions) {
-    final data = extensions.get<MessageDeliveryReceiptData>();
-    return data != null
-        ? [
-            data.toXML(),
-          ]
-        : [];
-  }
 }
 
 class MessageDeliveryReceivedData {
@@ -49,11 +41,6 @@ class MessageDeliveryReceivedData {
       attributes: {'id': id},
     );
   }
-
-  static List<XMLNode> messageSendingCallback(TypedMap extensions) {
-    final data = extensions.get<MessageDeliveryReceivedData>();
-    return data != null ? [data.toXML()] : [];
-  }
 }
 
 class MessageDeliveryReceiptManager extends XmppManagerBase {
@@ -116,4 +103,23 @@ class MessageDeliveryReceiptManager extends XmppManagerBase {
     );
     return state..done = true;
   }
+
+  List<XMLNode> _messageSendingCallback(TypedMap extensions) {
+    final data = extensions.get<MessageDeliveryReceivedData>();
+    return data != null
+        ? [
+            data.toXML(),
+          ]
+        : [];
+  }
+
+  @override
+  Future<void> postRegisterCallback() async {
+    await super.postRegisterCallback();
+
+    // Register the sending callback
+    getAttributes()
+        .getManagerById<MessageManager>(messageManager)
+        ?.registerMessageSendingCallback(_messageSendingCallback);
+  }
 }
diff --git a/packages/moxxmpp/lib/src/xeps/xep_0308.dart b/packages/moxxmpp/lib/src/xeps/xep_0308.dart
index 7621841..d38ddf8 100644
--- a/packages/moxxmpp/lib/src/xeps/xep_0308.dart
+++ b/packages/moxxmpp/lib/src/xeps/xep_0308.dart
@@ -2,6 +2,7 @@ import 'package:moxxmpp/src/managers/base.dart';
 import 'package:moxxmpp/src/managers/data.dart';
 import 'package:moxxmpp/src/managers/handlers.dart';
 import 'package:moxxmpp/src/managers/namespaces.dart';
+import 'package:moxxmpp/src/message.dart';
 import 'package:moxxmpp/src/namespaces.dart';
 import 'package:moxxmpp/src/stanza.dart';
 import 'package:moxxmpp/src/stringxml.dart';
@@ -22,15 +23,6 @@ class LastMessageCorrectionData {
       },
     );
   }
-
-  static List<XMLNode> messageSendingCallback(TypedMap extensions) {
-    final data = extensions.get<LastMessageCorrectionData>();
-    return data != null
-        ? [
-            data.toXML(),
-          ]
-        : [];
-  }
 }
 
 class LastMessageCorrectionManager extends XmppManagerBase {
@@ -64,4 +56,23 @@ class LastMessageCorrectionManager extends XmppManagerBase {
         LastMessageCorrectionData(edit.attributes['id']! as String),
       );
   }
+
+  List<XMLNode> _messageSendingCallback(TypedMap extensions) {
+    final data = extensions.get<LastMessageCorrectionData>();
+    return data != null
+        ? [
+            data.toXML(),
+          ]
+        : [];
+  }
+
+  @override
+  Future<void> postRegisterCallback() async {
+    await super.postRegisterCallback();
+
+    // Register the sending callback
+    getAttributes()
+        .getManagerById<MessageManager>(messageManager)
+        ?.registerMessageSendingCallback(_messageSendingCallback);
+  }
 }
diff --git a/packages/moxxmpp/lib/src/xeps/xep_0333.dart b/packages/moxxmpp/lib/src/xeps/xep_0333.dart
index 5858d20..99b6289 100644
--- a/packages/moxxmpp/lib/src/xeps/xep_0333.dart
+++ b/packages/moxxmpp/lib/src/xeps/xep_0333.dart
@@ -4,9 +4,11 @@ import 'package:moxxmpp/src/managers/base.dart';
 import 'package:moxxmpp/src/managers/data.dart';
 import 'package:moxxmpp/src/managers/handlers.dart';
 import 'package:moxxmpp/src/managers/namespaces.dart';
+import 'package:moxxmpp/src/message.dart';
 import 'package:moxxmpp/src/namespaces.dart';
 import 'package:moxxmpp/src/stanza.dart';
 import 'package:moxxmpp/src/stringxml.dart';
+import 'package:moxxmpp/src/util/typed_map.dart';
 
 class ChatMarkerData {
   const ChatMarkerData(this.isMarkable);
@@ -79,4 +81,19 @@ class ChatMarkerManager extends XmppManagerBase {
 
     return state..done = true;
   }
+
+  // TODO: Implement
+  List<XMLNode> _messageSendingCallback(TypedMap extensions) {
+    return [];
+  }
+
+  @override
+  Future<void> postRegisterCallback() async {
+    await super.postRegisterCallback();
+
+    // Register the sending callback
+    getAttributes()
+        .getManagerById<MessageManager>(messageManager)
+        ?.registerMessageSendingCallback(_messageSendingCallback);
+  }
 }
diff --git a/packages/moxxmpp/lib/src/xeps/xep_0334.dart b/packages/moxxmpp/lib/src/xeps/xep_0334.dart
index 5b372a6..401c83f 100644
--- a/packages/moxxmpp/lib/src/xeps/xep_0334.dart
+++ b/packages/moxxmpp/lib/src/xeps/xep_0334.dart
@@ -1,4 +1,10 @@
+import 'package:moxxmpp/src/managers/base.dart';
+import 'package:moxxmpp/src/managers/data.dart';
+import 'package:moxxmpp/src/managers/handlers.dart';
+import 'package:moxxmpp/src/managers/namespaces.dart';
+import 'package:moxxmpp/src/message.dart';
 import 'package:moxxmpp/src/namespaces.dart';
+import 'package:moxxmpp/src/stanza.dart';
 import 'package:moxxmpp/src/stringxml.dart';
 import 'package:moxxmpp/src/util/typed_map.dart';
 
@@ -46,8 +52,35 @@ enum MessageProcessingHint {
       xmlns: messageProcessingHintsXmlns,
     );
   }
+}
 
-  static List<XMLNode> messageSendingCallback(TypedMap extensions) {
+class MessageProcessingHintManager extends XmppManagerBase {
+  MessageProcessingHintManager() : super(messageProcessingHintManager);
+
+  @override
+  Future<bool> isSupported() async => true;
+
+  @override
+  List<StanzaHandler> getIncomingStanzaHandlers() => [
+        StanzaHandler(
+          stanzaTag: 'message',
+          tagXmlns: messageProcessingHintsXmlns,
+          callback: _onMessage,
+          // Before the message handler
+          priority: -99,
+        ),
+      ];
+
+  // TODO: Test
+  Future<StanzaHandlerData> _onMessage(
+    Stanza stanza,
+    StanzaHandlerData state,
+  ) async {
+    final element = stanza.findTagsByXmlns(messageProcessingHintsXmlns).first;
+    return state..extensions.set(MessageProcessingHint.fromName(element.tag));
+  }
+
+  List<XMLNode> _messageSendingCallback(TypedMap extensions) {
     final data = extensions.get<MessageProcessingHint>();
     return data != null
         ? [
@@ -55,4 +88,14 @@ enum MessageProcessingHint {
           ]
         : [];
   }
+
+  @override
+  Future<void> postRegisterCallback() async {
+    await super.postRegisterCallback();
+
+    // Register the sending callback
+    getAttributes()
+        .getManagerById<MessageManager>(messageManager)
+        ?.registerMessageSendingCallback(_messageSendingCallback);
+  }
 }
diff --git a/packages/moxxmpp/lib/src/xeps/xep_0359.dart b/packages/moxxmpp/lib/src/xeps/xep_0359.dart
index 4576efd..5cf561c 100644
--- a/packages/moxxmpp/lib/src/xeps/xep_0359.dart
+++ b/packages/moxxmpp/lib/src/xeps/xep_0359.dart
@@ -3,6 +3,7 @@ import 'package:moxxmpp/src/managers/base.dart';
 import 'package:moxxmpp/src/managers/data.dart';
 import 'package:moxxmpp/src/managers/handlers.dart';
 import 'package:moxxmpp/src/managers/namespaces.dart';
+import 'package:moxxmpp/src/message.dart';
 import 'package:moxxmpp/src/namespaces.dart';
 import 'package:moxxmpp/src/stanza.dart';
 import 'package:moxxmpp/src/stringxml.dart';
@@ -65,11 +66,6 @@ class StableIdData {
       if (stanzaIds != null) ...stanzaIds!.map((s) => s.toXML()),
     ];
   }
-
-  static List<XMLNode> messageSendingCallback(TypedMap extensions) {
-    final data = extensions.get<StableIdData>();
-    return data != null ? data.toXML() : [];
-  }
 }
 
 class StableIdManager extends XmppManagerBase {
@@ -126,4 +122,19 @@ class StableIdManager extends XmppManagerBase {
         ),
       );
   }
+
+  List<XMLNode> _messageSendingCallback(TypedMap extensions) {
+    final data = extensions.get<StableIdData>();
+    return data != null ? data.toXML() : [];
+  }
+
+  @override
+  Future<void> postRegisterCallback() async {
+    await super.postRegisterCallback();
+
+    // Register the sending callback
+    getAttributes()
+        .getManagerById<MessageManager>(messageManager)
+        ?.registerMessageSendingCallback(_messageSendingCallback);
+  }
 }
diff --git a/packages/moxxmpp/lib/src/xeps/xep_0385.dart b/packages/moxxmpp/lib/src/xeps/xep_0385.dart
index bbf66d0..6f03339 100644
--- a/packages/moxxmpp/lib/src/xeps/xep_0385.dart
+++ b/packages/moxxmpp/lib/src/xeps/xep_0385.dart
@@ -70,7 +70,9 @@ StatelessMediaSharingData parseSIMSElement(XMLNode node) {
   );
 }
 
+@Deprecated('Not maintained')
 class SIMSManager extends XmppManagerBase {
+  @Deprecated('Not maintained')
   SIMSManager() : super(simsManager);
 
   @override
diff --git a/packages/moxxmpp/lib/src/xeps/xep_0424.dart b/packages/moxxmpp/lib/src/xeps/xep_0424.dart
index 08e8bc5..4b3e8b8 100644
--- a/packages/moxxmpp/lib/src/xeps/xep_0424.dart
+++ b/packages/moxxmpp/lib/src/xeps/xep_0424.dart
@@ -2,6 +2,7 @@ import 'package:moxxmpp/src/managers/base.dart';
 import 'package:moxxmpp/src/managers/data.dart';
 import 'package:moxxmpp/src/managers/handlers.dart';
 import 'package:moxxmpp/src/managers/namespaces.dart';
+import 'package:moxxmpp/src/message.dart';
 import 'package:moxxmpp/src/namespaces.dart';
 import 'package:moxxmpp/src/stanza.dart';
 import 'package:moxxmpp/src/stringxml.dart';
@@ -15,37 +16,6 @@ class MessageRetractionData {
 
   /// The id of the message that is retracted.
   final String id;
-
-  static List<XMLNode> messageSendingCallback(TypedMap extensions) {
-    final data = extensions.get<MessageRetractionData>();
-    return data != null
-        ? [
-            XMLNode.xmlns(
-              tag: 'apply-to',
-              xmlns: fasteningXmlns,
-              attributes: <String, String>{
-                'id': data.id,
-              },
-              children: [
-                XMLNode.xmlns(
-                  tag: 'retract',
-                  xmlns: messageRetractionXmlns,
-                ),
-              ],
-            ),
-            if (data.fallback != null)
-              XMLNode(
-                tag: 'body',
-                text: data.fallback,
-              ),
-            if (data.fallback != null)
-              XMLNode.xmlns(
-                tag: 'fallback',
-                xmlns: fallbackIndicationXmlns,
-              ),
-          ]
-        : [];
-  }
 }
 
 class MessageRetractionManager extends XmppManagerBase {
@@ -92,4 +62,45 @@ class MessageRetractionManager extends XmppManagerBase {
         ),
       );
   }
+
+  List<XMLNode> _messageSendingCallback(TypedMap extensions) {
+    final data = extensions.get<MessageRetractionData>();
+    return data != null
+        ? [
+            XMLNode.xmlns(
+              tag: 'apply-to',
+              xmlns: fasteningXmlns,
+              attributes: <String, String>{
+                'id': data.id,
+              },
+              children: [
+                XMLNode.xmlns(
+                  tag: 'retract',
+                  xmlns: messageRetractionXmlns,
+                ),
+              ],
+            ),
+            if (data.fallback != null)
+              XMLNode(
+                tag: 'body',
+                text: data.fallback,
+              ),
+            if (data.fallback != null)
+              XMLNode.xmlns(
+                tag: 'fallback',
+                xmlns: fallbackIndicationXmlns,
+              ),
+          ]
+        : [];
+  }
+
+  @override
+  Future<void> postRegisterCallback() async {
+    await super.postRegisterCallback();
+
+    // Register the sending callback
+    getAttributes()
+        .getManagerById<MessageManager>(messageManager)
+        ?.registerMessageSendingCallback(_messageSendingCallback);
+  }
 }
diff --git a/packages/moxxmpp/lib/src/xeps/xep_0444.dart b/packages/moxxmpp/lib/src/xeps/xep_0444.dart
index 282ec74..dd85dc6 100644
--- a/packages/moxxmpp/lib/src/xeps/xep_0444.dart
+++ b/packages/moxxmpp/lib/src/xeps/xep_0444.dart
@@ -2,6 +2,7 @@ import 'package:moxxmpp/src/managers/base.dart';
 import 'package:moxxmpp/src/managers/data.dart';
 import 'package:moxxmpp/src/managers/handlers.dart';
 import 'package:moxxmpp/src/managers/namespaces.dart';
+import 'package:moxxmpp/src/message.dart';
 import 'package:moxxmpp/src/namespaces.dart';
 import 'package:moxxmpp/src/stanza.dart';
 import 'package:moxxmpp/src/stringxml.dart';
@@ -27,15 +28,6 @@ class MessageReactions {
       }).toList(),
     );
   }
-
-  static List<XMLNode> messageSendingCallback(TypedMap extensions) {
-    final data = extensions.get<MessageReactions>();
-    return data != null
-        ? [
-            data.toXML(),
-          ]
-        : [];
-  }
 }
 
 class MessageReactionsManager extends XmppManagerBase {
@@ -76,4 +68,23 @@ class MessageReactionsManager extends XmppManagerBase {
         ),
       );
   }
+
+  List<XMLNode> _messageSendingCallback(TypedMap extensions) {
+    final data = extensions.get<MessageReactions>();
+    return data != null
+        ? [
+            data.toXML(),
+          ]
+        : [];
+  }
+
+  @override
+  Future<void> postRegisterCallback() async {
+    await super.postRegisterCallback();
+
+    // Register the sending callback
+    getAttributes()
+        .getManagerById<MessageManager>(messageManager)
+        ?.registerMessageSendingCallback(_messageSendingCallback);
+  }
 }
diff --git a/packages/moxxmpp/lib/src/xeps/xep_0447.dart b/packages/moxxmpp/lib/src/xeps/xep_0447.dart
index be9b474..57aeb80 100644
--- a/packages/moxxmpp/lib/src/xeps/xep_0447.dart
+++ b/packages/moxxmpp/lib/src/xeps/xep_0447.dart
@@ -3,6 +3,7 @@ import 'package:moxxmpp/src/managers/base.dart';
 import 'package:moxxmpp/src/managers/data.dart';
 import 'package:moxxmpp/src/managers/handlers.dart';
 import 'package:moxxmpp/src/managers/namespaces.dart';
+import 'package:moxxmpp/src/message.dart';
 import 'package:moxxmpp/src/namespaces.dart';
 import 'package:moxxmpp/src/stanza.dart';
 import 'package:moxxmpp/src/stringxml.dart';
@@ -119,26 +120,6 @@ class StatelessFileSharingData {
           source is StatelessFileSharingUrlSource,
     ) as StatelessFileSharingUrlSource?;
   }
-
-  static List<XMLNode> messageSendingCallback(TypedMap extensions) {
-    final data = extensions.get<StatelessFileSharingData>();
-    if (data == null) {
-      return [];
-    }
-
-    // TODO(Unknown): Consider all sources?
-    final source = data.sources.first;
-    OOBData? oob;
-    if (source is StatelessFileSharingUrlSource && data.includeOOBFallback) {
-      // SFS recommends OOB as a fallback
-      oob = OOBData(source.url, null);
-    }
-
-    return [
-      data.toXML(),
-      if (oob != null) oob.toXML(),
-    ];
-  }
 }
 
 class SFSManager extends XmppManagerBase {
@@ -159,6 +140,26 @@ class SFSManager extends XmppManagerBase {
   @override
   Future<bool> isSupported() async => true;
 
+  List<XMLNode> _messageSendingCallback(TypedMap extensions) {
+    final data = extensions.get<StatelessFileSharingData>();
+    if (data == null) {
+      return [];
+    }
+
+    // TODO(Unknown): Consider all sources?
+    final source = data.sources.first;
+    OOBData? oob;
+    if (source is StatelessFileSharingUrlSource && data.includeOOBFallback) {
+      // SFS recommends OOB as a fallback
+      oob = OOBData(source.url, null);
+    }
+
+    return [
+      data.toXML(),
+      if (oob != null) oob.toXML(),
+    ];
+  }
+
   Future<StanzaHandlerData> _onMessage(
     Stanza message,
     StanzaHandlerData state,
@@ -170,4 +171,14 @@ class SFSManager extends XmppManagerBase {
         StatelessFileSharingData.fromXML(sfs),
       );
   }
+
+  @override
+  Future<void> postRegisterCallback() async {
+    await super.postRegisterCallback();
+
+    // Register the sending callback
+    getAttributes()
+        .getManagerById<MessageManager>(messageManager)
+        ?.registerMessageSendingCallback(_messageSendingCallback);
+  }
 }
diff --git a/packages/moxxmpp/lib/src/xeps/xep_0449.dart b/packages/moxxmpp/lib/src/xeps/xep_0449.dart
index efda5a5..be2da81 100644
--- a/packages/moxxmpp/lib/src/xeps/xep_0449.dart
+++ b/packages/moxxmpp/lib/src/xeps/xep_0449.dart
@@ -4,6 +4,7 @@ import 'package:moxxmpp/src/managers/base.dart';
 import 'package:moxxmpp/src/managers/data.dart';
 import 'package:moxxmpp/src/managers/handlers.dart';
 import 'package:moxxmpp/src/managers/namespaces.dart';
+import 'package:moxxmpp/src/message.dart';
 import 'package:moxxmpp/src/namespaces.dart';
 import 'package:moxxmpp/src/rfcs/rfc_4790.dart';
 import 'package:moxxmpp/src/stanza.dart';
@@ -236,22 +237,6 @@ class StickersData {
 
   /// The metadata of the sticker.
   final StatelessFileSharingData sticker;
-
-  static List<XMLNode> messageSendingCallback(TypedMap extensions) {
-    final data = extensions.get<StickersData>();
-    return data != null
-        ? [
-            XMLNode.xmlns(
-              tag: 'sticker',
-              xmlns: stickersXmlns,
-              attributes: {
-                'pack': data.stickerPackId,
-              },
-            ),
-            data.sticker.toXML(),
-          ]
-        : [];
-  }
 }
 
 class StickersManager extends XmppManagerBase {
@@ -285,6 +270,22 @@ class StickersManager extends XmppManagerBase {
       );
   }
 
+  List<XMLNode> _messageSendingCallback(TypedMap extensions) {
+    final data = extensions.get<StickersData>();
+    return data != null
+        ? [
+            XMLNode.xmlns(
+              tag: 'sticker',
+              xmlns: stickersXmlns,
+              attributes: {
+                'pack': data.stickerPackId,
+              },
+            ),
+            data.sticker.toXML(),
+          ]
+        : [];
+  }
+
   /// Publishes the StickerPack [pack] to the PubSub node of [jid]. If specified, then
   /// [accessModel] will be used as the PubSub node's access model.
   ///
@@ -350,4 +351,14 @@ class StickersManager extends XmppManagerBase {
 
     return Result(stickerPack);
   }
+
+  @override
+  Future<void> postRegisterCallback() async {
+    await super.postRegisterCallback();
+
+    // Register the sending callback
+    getAttributes()
+        .getManagerById<MessageManager>(messageManager)
+        ?.registerMessageSendingCallback(_messageSendingCallback);
+  }
 }
diff --git a/packages/moxxmpp/lib/src/xeps/xep_0461.dart b/packages/moxxmpp/lib/src/xeps/xep_0461.dart
index 95af407..0cb0b23 100644
--- a/packages/moxxmpp/lib/src/xeps/xep_0461.dart
+++ b/packages/moxxmpp/lib/src/xeps/xep_0461.dart
@@ -1,8 +1,10 @@
+import 'package:freezed_annotation/freezed_annotation.dart';
 import 'package:moxxmpp/src/jid.dart';
 import 'package:moxxmpp/src/managers/base.dart';
 import 'package:moxxmpp/src/managers/data.dart';
 import 'package:moxxmpp/src/managers/handlers.dart';
 import 'package:moxxmpp/src/managers/namespaces.dart';
+import 'package:moxxmpp/src/message.dart';
 import 'package:moxxmpp/src/namespaces.dart';
 import 'package:moxxmpp/src/stanza.dart';
 import 'package:moxxmpp/src/stringxml.dart';
@@ -51,44 +53,6 @@ class ReplyData {
 
     return body!.replaceRange(start!, end, '');
   }
-
-  static List<XMLNode> messageSendingCallback(TypedMap extensions) {
-    final data = extensions.get<ReplyData>();
-    return data != null
-        ? [
-            XMLNode.xmlns(
-              tag: 'reply',
-              xmlns: replyXmlns,
-              attributes: {
-                // The to attribute is optional
-                if (data.jid != null) 'to': data.jid!.toString(),
-
-                'id': data.id,
-              },
-            ),
-            if (data.body != null)
-              XMLNode(
-                tag: 'body',
-                text: data.body,
-              ),
-            if (data.body != null)
-              XMLNode.xmlns(
-                tag: 'fallback',
-                xmlns: fallbackXmlns,
-                attributes: {'for': replyXmlns},
-                children: [
-                  XMLNode(
-                    tag: 'body',
-                    attributes: {
-                      'start': data.start!.toString(),
-                      'end': data.end!.toString(),
-                    },
-                  ),
-                ],
-              ),
-          ]
-        : [];
-  }
 }
 
 /// Internal class describing how to build a message with a quote fallback body.
@@ -138,6 +102,45 @@ class MessageRepliesManager extends XmppManagerBase {
   @override
   Future<bool> isSupported() async => true;
 
+  @visibleForTesting
+  List<XMLNode> messageSendingCallback(TypedMap extensions) {
+    final data = extensions.get<ReplyData>();
+    return data != null
+        ? [
+            XMLNode.xmlns(
+              tag: 'reply',
+              xmlns: replyXmlns,
+              attributes: {
+                // The to attribute is optional
+                if (data.jid != null) 'to': data.jid!.toString(),
+
+                'id': data.id,
+              },
+            ),
+            if (data.body != null)
+              XMLNode(
+                tag: 'body',
+                text: data.body,
+              ),
+            if (data.body != null)
+              XMLNode.xmlns(
+                tag: 'fallback',
+                xmlns: fallbackXmlns,
+                attributes: {'for': replyXmlns},
+                children: [
+                  XMLNode(
+                    tag: 'body',
+                    attributes: {
+                      'start': data.start!.toString(),
+                      'end': data.end!.toString(),
+                    },
+                  ),
+                ],
+              ),
+          ]
+        : [];
+  }
+
   Future<StanzaHandlerData> _onMessage(
     Stanza stanza,
     StanzaHandlerData state,
@@ -167,4 +170,14 @@ class MessageRepliesManager extends XmppManagerBase {
         ),
       );
   }
+
+  @override
+  Future<void> postRegisterCallback() async {
+    await super.postRegisterCallback();
+
+    // Register the sending callback
+    getAttributes()
+        .getManagerById<MessageManager>(messageManager)
+        ?.registerMessageSendingCallback(messageSendingCallback);
+  }
 }
diff --git a/packages/moxxmpp/test/helpers/manager.dart b/packages/moxxmpp/test/helpers/manager.dart
index a3c02b7..b50381b 100644
--- a/packages/moxxmpp/test/helpers/manager.dart
+++ b/packages/moxxmpp/test/helpers/manager.dart
@@ -41,27 +41,31 @@ class TestingManagerHolder {
     return _managers[id] as T?;
   }
 
-  Future<void> register(XmppManagerBase manager) async {
-    manager.register(
-      XmppManagerAttributes(
-        sendStanza: _sendStanza,
-        getConnection: () => XmppConnection(
-          TestingReconnectionPolicy(),
-          AlwaysConnectedConnectivityManager(),
-          ClientToServerNegotiator(),
-          socket,
+  Future<void> register(List<XmppManagerBase> managers) async {
+    for (final manager in managers) {
+      manager.register(
+        XmppManagerAttributes(
+          sendStanza: _sendStanza,
+          getConnection: () => XmppConnection(
+            TestingReconnectionPolicy(),
+            AlwaysConnectedConnectivityManager(),
+            ClientToServerNegotiator(),
+            socket,
+          ),
+          getConnectionSettings: () => settings,
+          sendNonza: (_) {},
+          sendEvent: sentEvents.add,
+          getSocket: () => socket,
+          getNegotiatorById: getNegotiatorNullStub,
+          getFullJID: () => jid,
+          getManagerById: _getManagerById,
         ),
-        getConnectionSettings: () => settings,
-        sendNonza: (_) {},
-        sendEvent: sentEvents.add,
-        getSocket: () => socket,
-        getNegotiatorById: getNegotiatorNullStub,
-        getFullJID: () => jid,
-        getManagerById: _getManagerById,
-      ),
-    );
+      );
+      _managers[manager.id] = manager;
+    }
 
-    await manager.postRegisterCallback();
-    _managers[manager.id] = manager;
+    for (final manager in managers) {
+      await manager.postRegisterCallback();
+    }
   }
 }
diff --git a/packages/moxxmpp/test/xeps/xep_0030_test.dart b/packages/moxxmpp/test/xeps/xep_0030_test.dart
index f7ec19f..69f8d24 100644
--- a/packages/moxxmpp/test/xeps/xep_0030_test.dart
+++ b/packages/moxxmpp/test/xeps/xep_0030_test.dart
@@ -120,8 +120,7 @@ void main() {
       final ecm = EntityCapabilitiesManager('');
       final dm = DiscoManager([]);
 
-      await tm.register(dm);
-      await tm.register(ecm);
+      await tm.register([dm, ecm]);
 
       // Inject a capability hash into the cache
       final aliceJid = JID.fromString('alice@example.org/abc123');
diff --git a/packages/moxxmpp/test/xeps/xep_0060_test.dart b/packages/moxxmpp/test/xeps/xep_0060_test.dart
index 44d56a0..da31d14 100644
--- a/packages/moxxmpp/test/xeps/xep_0060_test.dart
+++ b/packages/moxxmpp/test/xeps/xep_0060_test.dart
@@ -55,8 +55,10 @@ void main() {
       () async {
     final manager = PubSubManager();
     final tm = TestingManagerHolder();
-    await tm.register(StubbedDiscoManager(false));
-    await tm.register(manager);
+    await tm.register([
+      StubbedDiscoManager(false),
+      manager,
+    ]);
 
     final result = await manager.preprocessPublishOptions(
       JID.fromString('pubsub.server.example.org'),
@@ -72,8 +74,10 @@ void main() {
       () async {
     final manager = PubSubManager();
     final tm = TestingManagerHolder();
-    await tm.register(StubbedDiscoManager(true));
-    await tm.register(manager);
+    await tm.register([
+      StubbedDiscoManager(true),
+      manager,
+    ]);
 
     final result = await manager.preprocessPublishOptions(
       JID.fromString('pubsub.server.example.org'),
diff --git a/packages/moxxmpp/test/xeps/xep_0115_test.dart b/packages/moxxmpp/test/xeps/xep_0115_test.dart
index 6febc7c..3daf321 100644
--- a/packages/moxxmpp/test/xeps/xep_0115_test.dart
+++ b/packages/moxxmpp/test/xeps/xep_0115_test.dart
@@ -319,8 +319,10 @@ void main() {
       final tm = TestingManagerHolder();
       final manager = EntityCapabilitiesManager('');
 
-      await tm.register(StubbedDiscoManager());
-      await tm.register(manager);
+      await tm.register([
+        StubbedDiscoManager(),
+        manager,
+      ]);
 
       await manager.onPresence(
         PresenceReceivedEvent(
@@ -352,8 +354,10 @@ void main() {
       final tm = TestingManagerHolder();
       final manager = EntityCapabilitiesManager('');
 
-      await tm.register(StubbedDiscoManager());
-      await tm.register(manager);
+      await tm.register([
+        StubbedDiscoManager(),
+        manager,
+      ]);
 
       await manager.onPresence(
         PresenceReceivedEvent(
@@ -385,10 +389,10 @@ void main() {
       final tm = TestingManagerHolder();
       final manager = EntityCapabilitiesManager('');
 
-      await tm.register(
+      await tm.register([
         StubbedDiscoManager()..multipleEqualIdentities = true,
-      );
-      await tm.register(manager);
+        manager,
+      ]);
 
       await manager.onPresence(
         PresenceReceivedEvent(
@@ -420,10 +424,10 @@ void main() {
       final tm = TestingManagerHolder();
       final manager = EntityCapabilitiesManager('');
 
-      await tm.register(
+      await tm.register([
         StubbedDiscoManager()..multipleEqualFeatures = true,
-      );
-      await tm.register(manager);
+        manager,
+      ]);
 
       await manager.onPresence(
         PresenceReceivedEvent(
@@ -455,10 +459,10 @@ void main() {
       final tm = TestingManagerHolder();
       final manager = EntityCapabilitiesManager('');
 
-      await tm.register(
+      await tm.register([
         StubbedDiscoManager()..multipleExtendedFormsWithSameType = true,
-      );
-      await tm.register(manager);
+        manager,
+      ]);
 
       await manager.onPresence(
         PresenceReceivedEvent(
@@ -490,10 +494,10 @@ void main() {
       final tm = TestingManagerHolder();
       final manager = EntityCapabilitiesManager('');
 
-      await tm.register(
+      await tm.register([
         StubbedDiscoManager()..invalidExtension1 = true,
-      );
-      await tm.register(manager);
+        manager,
+      ]);
 
       await manager.onPresence(
         PresenceReceivedEvent(
@@ -527,10 +531,10 @@ void main() {
       final tm = TestingManagerHolder();
       final manager = EntityCapabilitiesManager('');
 
-      await tm.register(
+      await tm.register([
         StubbedDiscoManager()..invalidExtension2 = true,
-      );
-      await tm.register(manager);
+        manager,
+      ]);
 
       await manager.onPresence(
         PresenceReceivedEvent(
@@ -564,10 +568,10 @@ void main() {
       final tm = TestingManagerHolder();
       final manager = EntityCapabilitiesManager('');
 
-      await tm.register(
+      await tm.register([
         StubbedDiscoManager()..invalidExtension3 = true,
-      );
-      await tm.register(manager);
+        manager,
+      ]);
 
       await manager.onPresence(
         PresenceReceivedEvent(
diff --git a/packages/moxxmpp/test/xeps/xep_0449_test.dart b/packages/moxxmpp/test/xeps/xep_0449_test.dart
index a322e7f..c58d972 100644
--- a/packages/moxxmpp/test/xeps/xep_0449_test.dart
+++ b/packages/moxxmpp/test/xeps/xep_0449_test.dart
@@ -234,10 +234,7 @@ void main() {
   });
 
   test('Test sending a sticker', () async {
-    final manager = MessageManager([
-      StatelessFileSharingData.messageSendingCallback,
-      StickersData.messageSendingCallback,
-    ]);
+    final manager = MessageManager();
     final holder = TestingManagerHolder(
       stubSocket: StubTCPSocket([
         StanzaExpectation(
@@ -265,7 +262,11 @@ void main() {
         ),
       ]),
     );
-    await holder.register(manager);
+    await holder.register([
+      manager,
+      StickersManager(),
+      SFSManager(),
+    ]);
 
     await manager.sendMessage2(
       JID.fromString('user@example.org'),
@@ -360,7 +361,7 @@ void main() {
         password: 'aaaa',
       );
     await conn.registerManagers([
-      MessageManager([]),
+      MessageManager(),
       SFSManager(),
       StickersManager(),
     ]);
diff --git a/packages/moxxmpp/test/xeps/xep_0461_test.dart b/packages/moxxmpp/test/xeps/xep_0461_test.dart
index 07df8b0..12da342 100644
--- a/packages/moxxmpp/test/xeps/xep_0461_test.dart
+++ b/packages/moxxmpp/test/xeps/xep_0461_test.dart
@@ -45,7 +45,7 @@ void main() {
   });
 
   test('Test calling the message sending callback', () {
-    final result = ReplyData.messageSendingCallback(
+    final result = MessageRepliesManager().messageSendingCallback(
       TypedMap()
         ..set(
           ReplyData.fromQuoteData(
@@ -129,7 +129,7 @@ void main() {
         password: 'aaaa',
       );
     await conn.registerManagers([
-      MessageManager([]),
+      MessageManager(),
       MessageRepliesManager(),
     ]);
     await conn.registerFeatureNegotiators([
@@ -228,7 +228,7 @@ void main() {
         password: 'aaaa',
       );
     await conn.registerManagers([
-      MessageManager([]),
+      MessageManager(),
       MessageRepliesManager(),
     ]);
     await conn.registerFeatureNegotiators([