Why not support the same versions of the OS that the makers support. For example, someone in this forum thinks that RHEL3 is an "old" kernel, however, it is not old. It is simply a different branch of the code. There are plenty of kernel updates going into the RHEL3 kernel and to not support it because it is old is not a good enough reason, IMHO.
From RedHat's website:
http://www.redhat.com/rhel/details/enterpriselinux3/
Red Hat Enterprise Linux v.3 was introduced in September 2003 and will be supported until August 2010
I think that would be enough of a reason to support RHEL3.
I would appreciate a supported version of Zimbra for RHEL3, very much!