http://www.zimbra.com/products/deskt...atibility.html says external client access through POP is supported:
"Clients such as Mozilla Thunderbird, Sunbird, and Eudora can be used with Zimbra to access email and even calendar data because the open source Zimbra Server uses an all standards-based approach and supports POP, IMAP, iCal, CalDAV, RSS, etc. The support is available in both the Open Source and Network Edition of Zimbra Collaboration Suite."
Maybe there is a server config setting that needs to be tweaked?