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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 This results in the following error:

Delivered to an SMTP server with IP address: (Not encrypted) Google tried to deliver your message, but it was rejected by the server for the recipient domain by []. 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?

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Generally, you need to indicate in the receiving mail server that the sending mail server is trusted (or whitelisted) which is why there is no password.

You need to look to see if Mailgun allows you to specify Google's mail servers as trusted for your mail.

If not, you could have the catch-all forward the mail to an unused account which then uses a filter to forward to an email address at the Mailgun server. Not pretty and subject to sending limits but might achieve your goal.

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