diff --git a/synapse/api/auth.py b/synapse/api/auth.py
index e05defd7d8..e2f40ee65a 100644
--- a/synapse/api/auth.py
+++ b/synapse/api/auth.py
@@ -63,7 +63,7 @@ class Auth(object):
             "user_id = ",
         ])
 
-    def check(self, event, auth_events):
+    def check(self, event, auth_events, do_sig_check=True):
         """ Checks if this event is correctly authed.
 
         Args:
@@ -79,6 +79,13 @@ 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)
+
+            # 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")
+
             if auth_events is None:
                 # Oh, we don't know what the state of the room was, so we
                 # are trusting that this is allowed (at least for now)
@@ -87,7 +94,6 @@ class Auth(object):
 
             if event.type == EventTypes.Create:
                 room_id_domain = get_domain_from_id(event.room_id)
-                sender_domain = get_domain_from_id(event.sender)
                 if room_id_domain != sender_domain:
                     raise AuthError(
                         403,
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 351b218247..4e8ffa8f7b 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -688,7 +688,7 @@ class FederationHandler(BaseHandler):
             logger.warn("Failed to create join %r because %s", event, e)
             raise e
 
-        self.auth.check(event, auth_events=context.current_state)
+        self.auth.check(event, auth_events=context.current_state, do_sig_check=False)
 
         defer.returnValue(event)
 
@@ -918,7 +918,7 @@ class FederationHandler(BaseHandler):
         )
 
         try:
-            self.auth.check(event, auth_events=context.current_state)
+            self.auth.check(event, auth_events=context.current_state, do_sig_check=False)
         except AuthError as e:
             logger.warn("Failed to create new leave %r because %s", event, e)
             raise e
diff --git a/synapse/state.py b/synapse/state.py
index d0f76dc4f5..d7d08570c9 100644
--- a/synapse/state.py
+++ b/synapse/state.py
@@ -379,7 +379,7 @@ class StateHandler(object):
             try:
                 # FIXME: hs.get_auth() is bad style, but we need to do it to
                 # get around circular deps.
-                self.hs.get_auth().check(event, auth_events)
+                self.hs.get_auth().check(event, auth_events, do_sig_check=False)
                 prev_event = event
             except AuthError:
                 return prev_event
@@ -391,7 +391,7 @@ class StateHandler(object):
             try:
                 # FIXME: hs.get_auth() is bad style, but we need to do it to
                 # get around circular deps.
-                self.hs.get_auth().check(event, auth_events)
+                self.hs.get_auth().check(event, auth_events, do_sig_check=False)
                 return event
             except AuthError:
                 pass