I hope this makes sense...
With our setup our mail server hostname is mail.domain.com and our zimbra was set up with that as the name. Our MX records point "domain.com" to this host so that email@example.com will be processed by our mail server.
Since the hostname is mail.domain.com, users are set up like: firstname.lastname@example.org and an alias is created for email@example.com so that mail to that address will be properly delivered.
I use nullmailer on other hosts in our network and sometimes sendmail to deliver cron messages and such. With other hosts I like to have mail show up as @domain.com instead of @host.domain.com so in nullmailer I set /etc/mailname to @domain.com and in sendmail I set up masquerading so that mail shows as @domain.com.
The problem I'm having is that mail to firstname.lastname@example.org gets rejected with a 550 error.
Jun 14 10:54:36 host nullmailer: smtp: Failed: 550 5.1.1 <email@example.com>: Recipient address rejected: domain.com.
However if I send to firstname.lastname@example.org it works. Is there an easy way to fix this? Sounds simple but I haven't found the answer yet.