... and your using ConfigServer's MailScanner package, look no further. I saw one of our servers getting strange undeliverable email, unroutable domain etc. email errors. After a little investigation I figured that cPanel had somehow itself (maybe during nightly update) switched BoxTrapper SpamTrap ON in tweak settings. Simply disable it and then remove /etc/eximmailtrap and your all set and email should work normally.

Thanks to Carlos @ TSS for hint to remove /etc/eximmailtrap