Pure curiosity drove me to graph the volume of email I've sent and received over the last decade. While I think the more interesting metric would be the average length of the emails, such a thing is impossible to measure accurately.1 That said, I think it's safe to say that my emails have become increasingly (and significantly) shorter over the years.
Regarding the graph below, the inputs for 1999 and (obviously) 2008 were not complete; I extrapolated the data I did have to come up with numbers for a full year. Also, it's probably worth mentioning that the volume of email shown includes only personal email (i.e., friend to friend); it does not cover emails from, for example, schools, financial institutions, employers, etc.
In an effort to give some context to the years, I can tell you that I was studying computer engineering from 1999 to 2003, and was in law school from 2003-2006.
Just think of the myriad mail clients people have used over the last decade, each of which handles replies, etc. differently; there simply is no way to globally sift out just the current body of every email. ↩