1) What version of Ubuntu? Zimbra is intended to work with the basic, no-frills 6.06 LTS server.
2) libdn may be different from libdn11. I would try apt-get install for all the packages that it says "missing" on.
Code:
apt-get install sudo libidn fetchmail gmp
Plus, if you haven't done so already (it won't warn you about this)
Code:
apt-get install perl
3) See
this wiki for detailed installation instructions. You MUST have DNS working before you do this install.
Cheers,
Dan