The SMTP standard is complicated and crazy. RJBS from Pobox does a good job of scaring anyone thinking of writing an SMTP client or server: <a href="http://www.youtube.com/watch?v=JENdgiAPD6c" rel="nofollow">http://www.youtube.com/watch?v=JENdgiAPD6c</a>
Interesting article. However, I would have liked if the author would have gone more in-depth on what type of applications could be build on top of SMTP by giving some examples and why SMTP would be more beneficial than e.g. HTTP for those applications.