diff --git a/federation/entities/base.py b/federation/entities/base.py
index 65f7cd3..792f7b1 100644
--- a/federation/entities/base.py
+++ b/federation/entities/base.py
@@ -133,6 +133,7 @@ class Profile(CreatedAtMixin, OptionalRawContentMixin, PublicMixin, BaseEntity):
     url = ""
     username = ""
     inboxes: Dict = None
+    mxid = ""
 
     _allowed_children = (Image,)
 
diff --git a/federation/entities/matrix/entities.py b/federation/entities/matrix/entities.py
index 8f6feb8..bbfdc9c 100644
--- a/federation/entities/matrix/entities.py
+++ b/federation/entities/matrix/entities.py
@@ -22,6 +22,9 @@ class MatrixEntityMixin(BaseEntity):
         # noinspection PyArgumentList
         return cls(**get_base_attributes(entity))
 
+    def get_endpoint(self, *args, **kwargs) -> str:
+        return "/_matrix/client/r0/"
+
     @property
     def txn_id(self) -> str:
         return self._txn_id
@@ -30,6 +33,13 @@ class MatrixEntityMixin(BaseEntity):
 class MatrixRoomMessage(Post, MatrixEntityMixin):
     _event_type = EventType.ROOM_MESSAGE.value
 
+    def get_endpoint(self, fid: str, user_id: str) -> str:
+        endpoint = super().get_endpoint()
+        return f"{endpoint}rooms/{fid}/send/{self.event_type}/{self.txn_id}?user_id={user_id}"
+
 
 class MatrixProfile(Profile, MatrixEntityMixin):
-    pass
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        # We always require an mxid
+        self._required.add('mxid')
diff --git a/federation/outbound.py b/federation/outbound.py
index b87bb91..cad68b9 100644
--- a/federation/outbound.py
+++ b/federation/outbound.py
@@ -321,7 +321,6 @@ def handle_send(
                 continue
             if not matrix_config:
                 matrix_config = get_matrix_configuration()
-            user_id = f"@{author_user.username}:{matrix_config['homeserver_name']}"
             # noinspection PyUnresolvedReferences
             payloads.append({
                 "auth": None,
@@ -331,8 +330,7 @@ def handle_send(
                 },
                 "payload": rendered_payload,
                 "urls": {
-                    f"{endpoint}/_matrix/client/r0/rooms/{fid}/send/{entity.event_type}/{entity.txn_id}"
-                    f"?user_id={user_id}",
+                    entity.get_endpoint(fid=fid, user_id=author_user.mxid),
                 },
             })
 
diff --git a/federation/types.py b/federation/types.py
index 81f05fa..afd3be2 100644
--- a/federation/types.py
+++ b/federation/types.py
@@ -28,6 +28,7 @@ class ReceiverVariant(Enum):
     FOLLOWERS = "followers"
 
 
+# TODO needed?
 class UserVariant(Enum):
     """
     Indicates whether the user is local or remote.
@@ -47,7 +48,8 @@ class UserType:
     guid: Optional[str] = attr.ib(default=None)
 
     # Required only if sending to Matrix protocol
-    username: Optional[str] = attr.ib(default=None)
+    mxid: Optional[str] = attr.ib(default=None)
+    # TODO needed?
     variant: Optional[UserVariant] = attr.ib(default=None)
 
     @property