Symantec 10.1.6 breaks SMTP

There's been a mysterious problem with users sending mail at work for about a week now and we finally figured out what the heck was happening on Friday. A new version of Symantec Antivirus, version 10.1.6.6010, occasionally manages to block all outbound connectivity from a Windows box to TCP port 25 (SMTP). This appears to be some bug in the feature that scans outgoing email for viruses. This Symantec forum post has a reference to this issue.