One of the most common things an Exchange administrator will want to do is find out how much mail their environment is sending out or receiving, and who is receving it or sending it, etc. Here are some options to solve the issue.

The ones that cost money

Quest MessageStats (I’ve never actually used it but Quest usually makes some very good tools)

http://www.quest.com/messagestats/microsoft-exchange.aspx

System Center Operations Manager (Using Exchange Management Pack)

http://www.microsoft.com/systemcenter/operationsmanager/en/us/default.aspx

The Ones That Are Free

These use the Message Tracking Logs and LogParser

Exchange 2007

http://msexchangeteam.com/archive/2007/11/12/447515.aspx

http://msexchangeteam.com/archive/2007/11/28/447598.aspx

http://msexchangeteam.com/archive/2008/02/07/448082.aspx

Exchange 2003

http://anewmessagehasarrived.blogspot.com/2008/03/exchange-2003-statistics-with-logparser.html