When having trouble with email I got the following error in the exim_paniclog on a cpanel server:

User 0 set for local_delivery transport is on the never_users list

The issue appears to be with some of the libraries the following commands should fix it:

ln -s libdb2.so.3 libdb.so.3
/scripts/eximup --force
/etc/rc.d/init.d/exim restart

Thanks to this thread on cpanel forums that I found after a little bit of searching for this fix:
http://forums.cpanel.net/showthread.php?p=135790