D. J. Bernstein
Internet mail

Mail disasters

Sendmail disasters
Postfix disasters

Mailers that can't handle temporary problems

Disclaimer: I'm the author of a free sendmail replacement, qmail. I wrote it because I was sick of sendmail's security holes.
``I sent e-mail to myself from five different remote accounts. Most letters arrived within two hours ... I discovered that five messages never made it. Three of them bounced, due to network problems or crashed computers along the way. The other two? Swallowed by the electronic abyss.'' Cliff Stoll, Silicon Snake Oil