The below has been known to fix most cPanel mail issues. They can range from spamd failed messages to exim just flat out not working at all.

/scripts/perlinstaller Digest::SHA1
/scripts/perlinstaller --force Mail::SpamAssassin
/scripts/fixspamassassinfailedupdate
/scripts/updatenow
/scripts/installspam --force
/scripts/exim4 --force

/etc/rc.d/init.d/exim restart
/scripts/restartsrv spamd
/etc/rc.d/init.d/chkservd restart