good article by Jon udell at
http://www.infoworld.com/article/04/03/12/11OPstrategic_1.html
Consider S/MIME digital signatures. It's very likely that your e-mail
client supports them.
But it's also very likely that someone's antivirus mail relay will
reject all mail with a .p7s signature attachment.
Maybe that's why eBay and Paypal aren't signing all their outbound mail.
Don't get me wrong; I think crypto is great. But to get there we still
have to go through a phase of designated sender schemes.