My curious habit is to count my email into two categories; read vs. unread. This was originally meant to be a measure of the pointless cc: / newsletter / internal announcement stuff I receive everyday; I have Outlook set to mark an eMail as Read only after I have opened it or looked in the Preview pane for more than 10 seconds (Tools, Options, Other, Preview Pane). Note this is running against my internal Outlook account, behind corporate firewall and Spam filter.
The graph shows the 700 days previous to Friday 9/10. It skips weekdays because I didn't get much internal eMail on the weekend (well, at least when I started) .
Anyhoo, the first signs of trouble were January 2003 - a marked uptick in unread mail due partially to a buggy corporate Spam filtering service. They got it working again, or so they thought, but in the summer of 2002, the amount of Spam that got through the corporate filter started rising steadily. This is roughly when the misspellings (Pain R.eli&ef meds sol%d here) and embedded text and other filter-defeating tricks started.
What I've noted is that, even though many say it's a security risk, I do appreciate the Preview pane that allows me to quickly scan eMail without spending too much time on it. The amount of "real work" spent with eMails (the Read Mail) has been pretty constant, and tools like Spambayes have helped shuffle off this trash out of my way.
The source for my counter is on my web site here (don't laugh, I'm just kicking off that page).