return 421 to retry when SMTPRecipientsRefused error

app/email/status.py

@@ -26,6 +26,7 @@ E402 = "421 SL E402 Encryption failed - Retry later"
 E404 = "421 SL E404 Unexpected error - Retry later"
 E405 = "421 SL E405 Mailbox domain problem - Retry later"
 E406 = "421 SL E406 Retry later"
+E407 = "421 SL E407 Retry later"
 
 # 5** errors
 E501 = "550 SL E501"

email_handler.py

@@ -835,7 +835,8 @@ def forward_email_to_mailbox(
         if should_ignore_bounce(envelope.mail_from):
             return True, status.E207
         else:
-            return False, status.E521
+            # can be Postfix intermittent "Temporary lookup failure" error
+            return False, status.E407
     else:
         Session.commit()
         return True, status.E200