diff --git a/clientapi/routing/register.go b/clientapi/routing/register.go
index f0ca372d9..614e19d50 100644
--- a/clientapi/routing/register.go
+++ b/clientapi/routing/register.go
@@ -329,12 +329,16 @@ func UserIDIsWithinApplicationServiceNamespace(
 	appservice *config.ApplicationService,
 ) bool {
 
-	var local, _, err = gomatrixserverlib.SplitID('@', userID)
+	var local, domain, err = gomatrixserverlib.SplitID('@', userID)
 	if err != nil {
 		// Not a valid userID
 		return false
 	}
 
+	if domain != cfg.Matrix.ServerName {
+		return false
+	}
+
 	if appservice != nil {
 		if appservice.SenderLocalpart == local {
 			return true