The most common cause of this is often spamhaus.org over-excitedly blocking. Fine in the case a repeat offender, but this often results in blocking mail servers so nothing that checks with spamhaus.org will accept your mail and there's a week long argument with the sysadmin before service is restored.
The mail server I use has hundreds of registered names (mail.com, doctor.com, etc.) and if one gets a spammer, the whole lot come down
. I'm intending to move, but I'm lazy.