We currently use Google Apps for receiving emails on our domain. We would like to send all mail to unknown addresses to our Mailgun server for further processing. This seems like a typical "split delivery" case.
I have attempted to create a custom destination (under "Email Routing" in the Google Apps settings) that sends to smtp.mailgun.com. This results in the following error:
Delivered to an SMTP server with IP address: 18.104.22.168 (Not encrypted)
Google tried to deliver your message, but it was rejected by the server for the recipient domain smtp.mailgun.org by smtp.mailgun.org. [22.214.171.124]. The error that the other server returned was: 550 5.7.1 Relaying denied
One thing that is suspicious, is that there is no setting to specify the email/password for the smtp server.
Does anyone know how to accomplish this?