I run a small personal email server using Zimbra and I noticed the stats are wrong as far as I know anyways.
for example the last 48 hours shows spikes of 30,000 messages.
and message volumes up into the 135,000,000 bytes.
I have a few domains and about half a dozen accounts, there is very little spam activity going and the server itself is just a dual core atom with 2GB of ram, I don't think it could handle 30,000 messages in a day
I am using ZCS 6 on Debian 64 bit.