Over the past couple weeks my Outlook 2007 clients have been randomly redownloading all of the old mail on the server. They are configured to connect via POP3 and to leave 30 days of mail on the server. It has only affected about 5% of my users but the ones it hits it tends to hit more than once.
I'm not really sure what is causing this, but I did recently upgrade Zimbra from 5.0.2 to 5.0.9 and also upgrade from Symantec Corporate AV 10.0 to Symantec Endpoint Protection 11.0 (settings were imported from the old management server so the scanning levels are the same). Both of these were done about 2 weeks before I started having this issue, so I'm not sure either are actually related.