diff --git a/synapse/api/auth.py b/synapse/api/auth.py
index 98a50f094..e75fd518b 100644
--- a/synapse/api/auth.py
+++ b/synapse/api/auth.py
@@ -72,7 +72,7 @@ class Auth(object):
         auth_events = {
             (e.type, e.state_key): e for e in auth_events.values()
         }
-        self.check(event, auth_events=auth_events, do_sig_check=False)
+        self.check(event, auth_events=auth_events, do_sig_check=do_sig_check)
 
     def check(self, event, auth_events, do_sig_check=True):
         """ Checks if this event is correctly authed.
@@ -91,11 +91,28 @@ class Auth(object):
             if not hasattr(event, "room_id"):
                 raise AuthError(500, "Event has no room_id: %s" % event)
 
-            sender_domain = get_domain_from_id(event.sender)
+            if do_sig_check:
+                sender_domain = get_domain_from_id(event.sender)
+                event_id_domain = get_domain_from_id(event.event_id)
 
-            # Check the sender's domain has signed the event
-            if do_sig_check and not event.signatures.get(sender_domain):
-                raise AuthError(403, "Event not signed by sending server")
+                is_invite_via_3pid = (
+                    event.type == EventTypes.Member
+                    and event.membership == Membership.INVITE
+                    and "third_party_invite" in event.content
+                )
+
+                # Check the sender's domain has signed the event
+                if not event.signatures.get(sender_domain):
+                    # We allow invites via 3pid to have a sender from a different
+                    # HS, as the sender must match the sender of the original
+                    # 3pid invite. This is checked further down with the
+                    # other dedicated membership checks.
+                    if not is_invite_via_3pid:
+                        raise AuthError(403, "Event not signed by sender's server")
+
+                # Check the event_id's domain has signed the event
+                if not event.signatures.get(event_id_domain):
+                    raise AuthError(403, "Event not signed by sending server")
 
             if auth_events is None:
                 # Oh, we don't know what the state of the room was, so we
@@ -491,6 +508,9 @@ class Auth(object):
         if not invite_event:
             return False
 
+        if invite_event.sender != event.sender:
+            return False
+
         if event.user_id != invite_event.user_id:
             return False
 
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index f7cb3c1bb..2d801bad4 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -1922,15 +1922,18 @@ class FederationHandler(BaseHandler):
             original_invite = yield self.store.get_event(
                 original_invite_id, allow_none=True
             )
-        if not original_invite:
+        if original_invite:
+            display_name = original_invite.content["display_name"]
+            event_dict["content"]["third_party_invite"]["display_name"] = display_name
+        else:
             logger.info(
-                "Could not find invite event for third_party_invite - "
-                "discarding: %s" % (event_dict,)
+                "Could not find invite event for third_party_invite: %r",
+                event_dict
             )
-            return
+            # We don't discard here as this is not the appropriate place to do
+            # auth checks. If we need the invite and don't have it then the
+            # auth check code will explode appropriately.
 
-        display_name = original_invite.content["display_name"]
-        event_dict["content"]["third_party_invite"]["display_name"] = display_name
         builder = self.event_builder_factory.new(event_dict)
         EventValidator().validate_new(builder)
         message_handler = self.hs.get_handlers().message_handler
diff --git a/synapse/types.py b/synapse/types.py
index 9d64e8c4d..1694af125 100644
--- a/synapse/types.py
+++ b/synapse/types.py
@@ -56,7 +56,7 @@ def get_domain_from_id(string):
     try:
         return string.split(":", 1)[1]
     except IndexError:
-        raise SynapseError(400, "Invalid ID: %r", string)
+        raise SynapseError(400, "Invalid ID: %r" % (string,))
 
 
 class DomainSpecificString(