diff --git a/changelog.d/5477.feature b/changelog.d/5477.feature
new file mode 100644
index 0000000000..63d9b58734
--- /dev/null
+++ b/changelog.d/5477.feature
@@ -0,0 +1 @@
+Allow server admins to define implementations of extra rules for allowing or denying incoming events.
diff --git a/synapse/events/third_party_rules.py b/synapse/events/third_party_rules.py
index ee7b97ad39..50ceeb1e8e 100644
--- a/synapse/events/third_party_rules.py
+++ b/synapse/events/third_party_rules.py
@@ -35,7 +35,10 @@ class ThirdPartyEventRules(object):
             module, config = hs.config.third_party_event_rules
 
         if module is not None:
-            self.third_party_rules = module(config=config)
+            self.third_party_rules = module(
+                config=config,
+                http_client=hs.get_simple_http_client(),
+            )
 
     @defer.inlineCallbacks
     def check_event_allowed(self, event, context):
@@ -81,3 +84,31 @@ class ThirdPartyEventRules(object):
         yield self.third_party_rules.on_create_room(
             requester, config, is_requester_admin
         )
+
+    @defer.inlineCallbacks
+    def check_threepid_can_be_invited(self, medium, address, room_id):
+        """Check if a provided 3PID can be invited in the given room.
+
+        Args:
+            medium (str): The 3PID's medium.
+            address (str): The 3PID's address.
+            room_id (str): The room we want to invite the threepid to.
+
+        Returns:
+            defer.Deferred[bool], True if the 3PID can be invited, False if not.
+        """
+
+        if self.third_party_rules is None:
+            defer.returnValue(True)
+
+        state_ids = yield self.store.get_filtered_current_state_ids(room_id)
+        room_state_events = yield self.store.get_events(state_ids.values())
+
+        state_events = {}
+        for key, event_id in state_ids.items():
+            state_events[key] = room_state_events[event_id]
+
+        ret = yield self.third_party_rules.check_threepid_can_be_invited(
+            medium, address, state_events,
+        )
+        defer.returnValue(ret)
diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index 93ac986c86..458902bb7e 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -72,6 +72,7 @@ class RoomMemberHandler(object):
 
         self.clock = hs.get_clock()
         self.spam_checker = hs.get_spam_checker()
+        self.third_party_event_rules = hs.get_third_party_event_rules()
         self._server_notices_mxid = self.config.server_notices_mxid
         self._enable_lookup = hs.config.enable_3pid_lookup
         self.allow_per_room_profiles = self.config.allow_per_room_profiles
@@ -723,6 +724,15 @@ class RoomMemberHandler(object):
         # can't just rely on the standard ratelimiting of events.
         yield self.base_handler.ratelimit(requester)
 
+        can_invite = yield self.third_party_event_rules.check_threepid_can_be_invited(
+            medium, address, room_id,
+        )
+        if not can_invite:
+            raise SynapseError(
+                403, "This third-party identifier can not be invited in this room",
+                Codes.FORBIDDEN,
+            )
+
         invitee = yield self._lookup_3pid(
             id_server, medium, address
         )