here's somewhere to start:
http://www.zimbra.com/pdf/Zimbra%20S...0Benchmark.pdf
Disclaimer: I'm not the perf master, just want to give you a starting point. I'm sure others will chime in.
The setup described in the document sounds like overkill for your needs. The machines described in the doc or something equivalent should be more than adequate and give you plenty of room for growth.
Dual-CPU Dual-core AMD Opteron 280 2.4GHz (4 logical CPUs)
8 GB RAM
For a loaded 50K users machine with average mailbox size of 30MB, your users will have about 1.5TB of mail, which is somewhere in the 3-3.5TB of disk space across the message store, database and index store. Based on your performance & reliability requirements, if you are going to use clustering, if you have existing storage hardware (SAN) etc, you'll need to find the best storage solution. As a tip, if you have the freedom to choose, more smaller drives are better than fewer bigger ones as they can do more in parallel.
Sound like for your workload, you could do it all with a single server (this is a hunch, not a result of performance testing).