diff --git a/tests/smtpd.py b/tests/smtpd.py
index b0e1ac30..79ddaddd 100644
--- a/tests/smtpd.py
+++ b/tests/smtpd.py
@@ -115,6 +115,8 @@ async def smtp_AUTH(self, arg):
 
         if arg[:5] == "LOGIN":
             await self.smtp_AUTH_LOGIN(arg[6:])
+        else:
+            await self.push("504 Unsupported AUTH mechanism.")
 
     async def smtp_AUTH_LOGIN(self, arg):
         username = base64.b64decode(arg)