diff --git a/synapse/api/auth.py b/synapse/api/auth.py
index 9c207b9537..6a97c06110 100644
--- a/synapse/api/auth.py
+++ b/synapse/api/auth.py
@@ -809,11 +809,8 @@ class Auth(object):
             elif threepid:
                 # If the user does not exist yet, but is signing up with a
                 # reserved threepid then pass auth check
-                for tp in self.hs.config.mau_limits_reserved_threepids:
-                    if (threepid['medium'] == tp['medium']
-                            and threepid['address'] == tp['address']):
-                        return
-
+                if is_threepid_reserved(threepid):
+                    return
             # Else if there is no room in the MAU bucket, bail
             current_mau = yield self.store.get_monthly_active_count()
             if current_mau >= self.hs.config.max_mau_value:
diff --git a/synapse/rest/client/v1_only/register.py b/synapse/rest/client/v1_only/register.py
index 2c7bbcb171..95873e03d5 100644
--- a/synapse/rest/client/v1_only/register.py
+++ b/synapse/rest/client/v1_only/register.py
@@ -291,6 +291,10 @@ class RegisterRestServlet(ClientV1RestServlet):
             password=password,
             threepid=threepid,
         )
+        # Necessary due to auth checks prior to the threepid being
+        # written to the db
+        if self.store.is_threepid_reserved(threepid):
+            self.store.upsert_monthly_active_user(registered_user_id)
 
         if session[LoginType.EMAIL_IDENTITY]:
             logger.debug("Binding emails %s to %s" % (
diff --git a/synapse/rest/client/v2_alpha/register.py b/synapse/rest/client/v2_alpha/register.py
index 45113e5386..f22b7577ea 100644
--- a/synapse/rest/client/v2_alpha/register.py
+++ b/synapse/rest/client/v2_alpha/register.py
@@ -406,6 +406,10 @@ class RegisterRestServlet(RestServlet):
                 generate_token=False,
                 threepid=threepid,
             )
+            # Necessary due to auth checks prior to the threepid being
+            # written to the db
+            if self.store.is_threepid_reserved(threepid):
+                self.store.upsert_monthly_active_user(registered_user_id)
 
             # remember that we've now registered that user account, and with
             #  what user ID (since the user may not have specified)
diff --git a/synapse/storage/monthly_active_users.py b/synapse/storage/monthly_active_users.py
index d178f5c5ba..173867c4b1 100644
--- a/synapse/storage/monthly_active_users.py
+++ b/synapse/storage/monthly_active_users.py
@@ -36,7 +36,6 @@ class MonthlyActiveUsersStore(SQLBaseStore):
 
     @defer.inlineCallbacks
     def initialise_reserved_users(self, threepids):
-        # TODO Why can't I do this in init?
         store = self.hs.get_datastore()
         reserved_user_list = []
 
@@ -220,3 +219,17 @@ class MonthlyActiveUsersStore(SQLBaseStore):
                     yield self.upsert_monthly_active_user(user_id)
             elif now - last_seen_timestamp > LAST_SEEN_GRANULARITY:
                 yield self.upsert_monthly_active_user(user_id)
+
+    def is_threepid_reserved(self, threepid):
+        """Check the threepid against the reserved threepid config
+        Args:
+            threepid(dict) - The threepid to test for
+        Returns:
+            boolean Is the threepid undertest reserved_user
+        """
+        for tp in self.hs.config.mau_limits_reserved_threepids:
+            if (threepid['medium'] == tp['medium']
+                    and threepid['address'] == tp['address']):
+                return True
+            else:
+                return False