Building Zimbra using Perforce - Zimbra :: Wiki also contains how to obtain the source code.
First grab a perforce client:
Perforce Downloads: P4 Downloads
Your connecting to codes.zimbra.com:2666
(Other configuration as needed & outlined in
Franklin README - Zimbra :: Wiki)
Example command with user/pass:
p4 -u public -P public1234 -c public-view sync -f //depot/zcs/FRANKLIN/...@RELEASE_5.0.5_GA
or ...@2008/04/17:00:00:00
And you have a copy of the source in your target location.