Outlook 2007 + Spam Blocker or Virus Blocker + With Protocol Control Enabled
Email messages get borked while trying to transmit/receive them. It works fine under SSL, but that bypasses the filter.
This only happens when using Outlook 2007 with either the spam blocker or the virus blocker enabled (or both) while the protocol control is enabled. Outlook 2003 works fine under the same conditions, so something with outlook 2007 does not play well with this combination.
The only solution was to disable protocol control.
From reading around the forum, it is a common problem.