We had just changed a domain from ABC to XYZ for a client.
Shortly after all email stopped working on the server and we started getting tons of noise in our exim logs:
Somehow Cpanel had completely erased the /etc/userdomains file. This is a critical file for your email, Exim uses it to authenticate your logins and without it, no one can send or receive email.
Cpanel automated backup does NOT backup this file - you must do it manually.
The /etc/userdomains looks like this:
yourdomain.com: username
abcdomain.com: username
After some panic this morning we restore the file with a handy script: /scripts/updateuserdomains
Although I've heard it doesn't always work so be sure to backup the file just to be safe. If neither works then you will need to have a copy of all the domains hosted with each accounts username then manually add them back into the /etc/userdomains which can take hours.
Cheers,