Dovecot

[rbtux]

The error Domain Not Found (Sender Address rejected) is because there is no a or mx dns record for the domain of the sending mail address. (this can also be caused by wrong dns resolver configuration, but I'm not sure if postfix would use a 450 error code then...)

Verify the domain on the server using "host domain.co.uk".

The postfix parameter that causes that (correct) behavior is reject_unknown_sender_domain unter smtpd_*_restrictions in your /etc/postfix/main.cf