if it works and there's an open source version for Ubuntu, and Ubuntu is a well known commercial distro, Why do you say "it's not supported"?
Why don't you say "We won't support it" or "we can't support it" or even "we don't want to support it"?
By the way, I found this:
Operating System Network Edition
• Red Hat® Enterprise Linux®, AS/ES version 4. (32-bit, 64-bit)
• Mac OS® X 10.4.7
• SUSE ES 9 (32-bit) and Open SUSE 10
• Ubuntu 6.06 (as of 4.5.4)
• rPath VMware (beta)
http://files.zimbra.com/website/docs...quirements.pdf