JBMJBM has released eMail Stats 1.0 (http://www.jbmjbm.com/emailstats.html) for Mac OS X 10.6 or higher. The app monitors your daily, weekly, monthly, and yearly email usage. It costs US$1.99 and is available through the Mac App Store in the Utilities category.
If you ever wonder how many emails you have been sending or receiving, this app can tell you. As a matter of fact, it has your email statistics for the day, previous day, week, month, and the year and all from your Mac Menu Bar.