If you are using the Mac OS X Mail application, as I still do on my laptop, you will probably have noticed the flurry of spam emails in recent months purporting to come from your own email address. I think that individual cryptographic certificates are one tool for tackling spam email in general, but a problem that I had experienced was how to reject emails that say that they come from me but which don't have a digital certificate, because there is no clear way to reject unsigned emails in the existing rules and I don't have enough applescript foo to be able to remedy this. I have found though that you can turn off the automatic white-listing of your own email address under the Junk Mail pane of the Mail App. preferences. That, inconjunction with a rule whitelisting my
certificated email, should hopefully be enough for now.