We're running Courier 0.57 authlib, mta, imap, etc... on RHEL4 on a dual proc Xeon server. We're also running ClamAV through ClamCour, and hitting SpamAssassing through a maildrop xfilter. On average, the load on the system is very, very low - < 0.05

Lately, we've started to see a bunch of flaky behaviours, where users would be prompted for their passwords on send (we're using authenticated, and SSL smtp) randomly. We've also had situations where messages show up in the users sent folder, but are never delivered. In addition, we've seen situations where saving to the sent folder has failed. This one we've mostly cleared up by allowing more connections per IP, as we all appear to be coming from the same IP to the server, since it's outside our corporate firewall.

I'm getting a lot of pressure to dump courier at this point, and I'm starting to think that might be the right way to go. There's nothing useful in the logs - apparently only things that work get logged ;) None of the failures have corresponding messages.

Our clients are all running Thunderbird on Windows, OSX and Linux.

Any thoughts or advice on how to debug these types of sporadic issues?