I administer a few dozen boxes for our organization. Mostly Debian, a few Gentoo and a RHEL box running Zimbra. SSMTP is used on all boxes to get root's emails in one place - Zimbra. From there I setup my local web based email client (Squirrelmail) to "POP3 Fetch Mail". I set up a few dozen accounts on Zimbra, a few dozen folders in SM and pop away, read emails, do what sysadmins do. Everything is grand for a couple years. Now all of a sudden....
Some Logwatch emails cause the client to stop dead ""dropped by server" and Zimbra logs show client disconnected w/out quit. I'm sure these are generic messages for a timeout. Tried increasing the verbosity for individual accounts on Zimbra without much insight as to where the problem is emanating from. Deleting offending emails via Zimbra's web interface allows the poping to continue. Checked all firewalls, even tried a windoz box w/Outlook 03 in pop mode with the same results. In IMAP mode Outlook worked fine testing on one account, so I'm finding it hard to guess this is a clamav issue blocking a string or something and leading me to suspect the POP3 server portion of Zimbra but have never seen this kind of behavior in 20 years, the logs are providing no clues where to go next.