postfix/smtp: invalid sender domain (misconfigured dns?) (in reply to RCPT TO command)

A few days ago, we switched a newsletter function on our new server and got the following error messages in syslog:

Mar 28 18:17:50 xxxxxx postfix/smtp[29479]: 65EA137AE460:
host xxx.xxx.com[nnn.nnn.nnn.nnn] said: 421 invalid sender domain
'yyy.yyy.com' (misconfigured dns?) (in reply to RCPT TO command)


Mar 28 18:17:50 xxxxxx postfix/smtp[29479]: 65EA137AE460:
to=<zzz@zzzzzz.com>,
relay=xxx.xxx.com[nnn.nnn.nnn.nnn]:25, delay=0.51,
delays=0/0/0.26/0.24, dsn=4.0.0, status=deferred (host
xxx.xxx.com[nnn.nnn.nnn.nnn] said: 421 invalid sender domain
'yyy.yyy.com' (misconfigured dns?) (in reply to RCPT TO
command))

The problem was that when our server was installed the mail name (in /etc/mailname) was set to some defaults (in our case debian.vaultronserver.de) which doesn't match our hostname or any DNS entry related to our server. It should actually have been the fully qualified name of our server.


The problem was quickly solved by updating /etc/mailname:

hostname -A > /etc/mailname

And restarting postfix:

/etc/init.d/postfix restart