Nasty. Go to command line, issue /sbin/e-smith/signal-event post-upgrade, then
/sbin/e-smith/signal-event reboot
After reboot change to /var/logs. Type in console
tail -f messages
Try to force the error with you webmail and look, if there is something strange happening in the message log. Repeat that for other logs like maillog, qmail. DON'T post a load of logs here.
You can also use server manager to look at logs, but there is no real time update like the tail command. Report back.