Hopefully someone with knowledge will answer you here soon. But until they do..... give your local Sales people a call http://www.mcafee.com/mx/products/email-gateway.aspx
(In Mexico I believe see the phone number on that page).
These questions are tough and are always difficult to explain.
With only Anti-Spam and Anti-Virus its about 192,000 per hour. This includes messages that are dropped at the connection level.
With Anti-Spam, Anti-Virus, and Encryption its about 140,000 per hour.
The key factors here are what your average message size is, and how many bells and whistles you want to enable. The three major ones are Anti-Spam, Anti-Virus, and encryption.
If your average message size is over 1 meg (which would be impressive and would probably indicate that you would want to consider an FTP server also) its about probably 60,000 per hour.
If you plan on using it as a pop/imap proxy with an average message size over 1 meg its probably about 40,000 per hour.
This is assuming you are running it in explicit proxy mode.