diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 63c56a4a3..7711cded0 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -122,6 +122,28 @@ class FederationHandler(BaseHandler):
             self.room_queues[pdu.room_id].append((pdu, origin))
             return
 
+        # If we're no longer in the room just ditch the event entirely. This
+        # is probably an old server that has come back and thinks we're still
+        # in the room (or we've been rejoined to the room by a state reset).
+        #
+        # If we were never in the room then maybe our database got vaped and
+        # we should check if we *are* in fact in the room. If we are then we
+        # can magically rejoin the room.
+        is_in_room = yield self.auth.check_host_in_room(
+            pdu.room_id,
+            self.server_name
+        )
+        if not is_in_room:
+            was_in_room = yield self.store.was_host_joined(
+                pdu.room_id, self.server_name,
+            )
+            if was_in_room:
+                logger.info(
+                    "Ignoring PDU %s for room %s from %s as we've left the room!",
+                    pdu.event_id, pdu.room_id, origin,
+                )
+                return
+
         state = None
 
         auth_chain = []
diff --git a/synapse/storage/events.py b/synapse/storage/events.py
index 7002b3752..4f0b43c36 100644
--- a/synapse/storage/events.py
+++ b/synapse/storage/events.py
@@ -784,6 +784,9 @@ class EventsStore(SQLBaseStore):
                     self._invalidate_cache_and_stream(
                         txn, self.is_host_joined, (room_id, host)
                     )
+                    self._invalidate_cache_and_stream(
+                        txn, self.was_host_joined, (room_id, host)
+                    )
 
                 self._invalidate_cache_and_stream(
                     txn, self.get_users_in_room, (room_id,)
diff --git a/synapse/storage/roommember.py b/synapse/storage/roommember.py
index 457ca288d..a0fc9a686 100644
--- a/synapse/storage/roommember.py
+++ b/synapse/storage/roommember.py
@@ -533,6 +533,46 @@ class RoomMemberStore(SQLBaseStore):
 
         defer.returnValue(True)
 
+    @cachedInlineCallbacks()
+    def was_host_joined(self, room_id, host):
+        """Check whether the server is or ever was in the room.
+
+        Args:
+            room_id (str)
+            host (str)
+
+        Returns:
+            Deferred: Resolves to True if the host is/was in the room, otherwise
+            False.
+        """
+        if '%' in host or '_' in host:
+            raise Exception("Invalid host name")
+
+        sql = """
+            SELECT user_id FROM room_memberships
+            WHERE room_id = ?
+                AND user_id LIKE ?
+                AND membership = 'join'
+            LIMIT 1
+        """
+
+        # We do need to be careful to ensure that host doesn't have any wild cards
+        # in it, but we checked above for known ones and we'll check below that
+        # the returned user actually has the correct domain.
+        like_clause = "%:" + host
+
+        rows = yield self._execute("was_host_joined", None, sql, room_id, like_clause)
+
+        if not rows:
+            defer.returnValue(False)
+
+        user_id = rows[0][0]
+        if get_domain_from_id(user_id) != host:
+            # This can only happen if the host name has something funky in it
+            raise Exception("Invalid host name")
+
+        defer.returnValue(True)
+
     def get_joined_hosts(self, room_id, state_entry):
         state_group = state_entry.state_group
         if not state_group: