There's some community Mac OS X 10.4 PPC builds here:
https://sourceforge.net/project/show...kage_id=273119
(That's not the only site to find community releases, but dijichi2 does a lot with Ubuntu & Solaris as well there.)
To compile your own:
Building Zimbra using Perforce - Zimbra :: Wiki &
Franklin README - Zimbra :: Wiki
Network Editions - Officially tested & technical support by Zimbra.
Open Source Editions - Built by Zimbra - forum help only.
Community Editions - Built by members of this excellent community!