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501 Sender address rejected: User unknown When external sender is also a local user

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I have an iRedMail Server. It has a domain (example.com) with [email protected] as a registered user. I can send to and receive from [email protected]. from my personal account [email protected] so I know the user is properly setup on the server.

However - [email protected] is the sending address / admin for the website. example.com Originally I was going to set up auth SMTP access to the mail server and send from the website via the mail server using authenticated SMTP. but decided to run with MailGun.

So [email protected], sends out emails, including to other users @example.com via mail gun.

Any email from [email protected], via mailgun, to any user on the server (even other domains on the server) get a 501 Sender address rejected: User unknown.

It works perfectly to any other domain on other servers. It works perfectly if I am smtp authenticated on the server but not if an external smtp server sends email on behalf. SPF, DKIM, DMARC etc all set up correctly.

Its a Postfix background on the iRedMail server.

mangohost

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