[shanson]

I missed the suggestion from from dkarp back in June. That's too bad, since the problem persisted even after updating to 4.x, and I resubmitted the problem to Zimbra Support. The second thing they suggested was checking zimbraImapNumThreads.

We have about 120 users, almost all of whom use IMAP. The default value of 200 IMAP threads is therefore most likely too small. We've increased to 400, we'll see if the problem reoccurs.