For an interesting presentation on the complexity of e-mail check out the following presentation:

http://video.google.com/videoplay?docid=7054401183589794595#

Until I saw this, I was unaware of how badly designed the current e-mail structure is, and how badly/incomplete the RFCs for e-mail are. Unfortunately the slides are blurry so you cannot see the examples very well but the talk alone is very informative.