Zimbra does require more resources than most mail servers (minimum 2 GB of RAM), but it has a superb interface.
It can work in any size environment - from just a few users, to a few million users. So to answer your question, overkill would need to be defined based on what your requirements are.
If you want to use Zimbra for your email but do not want to run your own Zimbra server, there are plenty of hosts out there that will gladly sell you Zimbra email accounts, also
