I'm sysadmin of a small (9 user) domain running Windows 2003 Small Business Server (SBS). Email is handled by Exchange. Server is 3GHz, 4GB RAM, RAID5, etc. Internet connection is broadband.
Our incoming and outgoing email has been operating without any significant problems for over 2 years. Suddenly, about a week or so ago, we started receiving <delayed> and < not delivered> messages around 24-48 hours after an email message is sent out. One user, in particular, is having most of her outgoing messages undelivered. Some other users have occasional undelivered messages, and some are having no problem at all.
We are running Norton Corp Anti-Virus v10, which scans all outgoing messages, so delays and rejections from virus scanners are unlikely (though possible).
How would I go about tracking down the source of the problems?