diff --git a/synapse/federation/federation_server.py b/synapse/federation/federation_server.py
index 510a17682..bc20b9c20 100644
--- a/synapse/federation/federation_server.py
+++ b/synapse/federation/federation_server.py
@@ -146,11 +146,15 @@ class FederationServer(FederationBase):
             # check that it's actually being sent from a valid destination to
             # workaround bug #1753 in 0.18.5 and 0.18.6
             if transaction.origin != get_domain_from_id(pdu.event_id):
+                # We continue to accept join events from any server; this is
+                # necessary for the federation join dance to work correctly.
+                # (When we join over federation, the "helper" server is
+                # responsible for sending out the join event, rather than the
+                # origin. See bug #1893).
                 if not (
                     pdu.type == 'm.room.member' and
                     pdu.content and
-                    pdu.content.get("membership", None) == 'join' and
-                    self.hs.is_mine_id(pdu.state_key)
+                    pdu.content.get("membership", None) == 'join'
                 ):
                     logger.info(
                         "Discarding PDU %s from invalid origin %s",
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 888dd0124..2ecc0087b 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -1004,9 +1004,19 @@ class FederationHandler(BaseHandler):
         )
 
         event.internal_metadata.outlier = False
-        # Send this event on behalf of the origin server since they may not
-        # have an up to data view of the state of the room at this event so
-        # will not know which servers to send the event to.
+        # Send this event on behalf of the origin server.
+        #
+        # The reasons we have the destination server rather than the origin
+        # server send it are slightly mysterious: the origin server should have
+        # all the neccessary state once it gets the response to the send_join,
+        # so it could send the event itself if it wanted to. It may be that
+        # doing it this way reduces failure modes, or avoids certain attacks
+        # where a new server selectively tells a subset of the federation that
+        # it has joined.
+        #
+        # The fact is that, as of the current writing, Synapse doesn't send out
+        # the join event over federation after joining, and changing it now
+        # would introduce the danger of backwards-compatibility problems.
         event.internal_metadata.send_on_behalf_of = origin
 
         context, event_stream_id, max_stream_id = yield self._handle_new_event(