Sending mail service unavailable status

Question

syslog is consistently returning for a specific domain the following result:

relay=mail.domain.tld. [XX.XXX.XXX.XX], dsn=5.0.0, stat=Service unavailable

what is a possible cause of the service unavailability and how can this be resolved?

Answer 1

sendmail (by sendmail.org) - getting outgoing SMTP transcript

As root send a test message with SMTP session tracking. It may provide more hints.

#!/bin/sh 
TO=recipient@domain.tld
FROM=someone@your.domain

/usr/sbin/sendmail -v -Ac -i -f$FROM -- $TO <<END
subject: test
to: $TO
from: $FROM

test
END