I recently collected stats on SMTP traffic. I parsed the console logs (I had a little over six months worth of console logs to parse) for "SMTP Server: Message ", outputting the results to a text file. I then opened the file in Excel, and copied the date column to another worksheet. I then used Excel's subtotal function to find what the peak day for traffic was, and compare this to the average for the whole period. Since one Domino server does most of the transfers to the outbound Internet gateways, this was not as time consuming as it may sound.
It would be nice if the max and average amount of traffic could be displayed on an ongoing basis. Enabling billing does not, if I recall correctly, distinguish between NRPC (internal) and SMTP mail traffic.