3 December 2013

Debian / Ubuntu Postfix Error: Diagnostic-Code: X-Postfix; unknown user:

I've just set up a new Debian/Ubuntu server, and had a problem with Postfix rejecting mail.  Mails were getting returned to sender with the error:

Diagnostic-Code: X-Postfix; unknown user: "admin"

From the command line, I opened the Postfix config file:

vi /etc/postfix/main.cf

Then I found the line:

mydestination = example.com, localhost, localhost.localdomain

(where example.com was the server address) and changed it to:

mydestination = localhost, localhost.localdomain

I saved the file (:wq) and restarted Postfix:

service postfix retstart

Emails started arriving! I believe that the original format confuses Postfix- instead of trying to find and deliver to the local mailboxes, it tries to forward them on to example.com perhaps?