E-mail is many things, but, unfortunately, “safe” is not one of them.
There is an old saw that you should never send something in e-mail that you would mind seeing on the front page of the New York Times, and that’s as true today as it was when e-mail first became popular in the 90′s. E-mail (and, in fact, all Internet traffic) travels through many servers between your computer and its final destination, all belonging to strangers. And while it’s true that the sheer volume of e-mail that travels the ‘net these days is probably enough to keep yours safe even if someone does care enough to snoop, I still wouldn’t send a credit card number or the PIN for my ATM card through e-mail.
This is a long-standing problem that has yet to be fully addressed by standards, but there is a solution, albeit one that requires you and your e-mail partner to take some steps. You can get a free digital certificate that will allow you to digitally “sign” your e-mails and allow others to send you encrypted e-mail. You can also send encrypted e-mail to anyone who sends their certificate to you.
This all sounds more complicated than it is. Basically, the steps are to sign up for a certificate, install that certificate (handled automatically for you by the Mac), then start using it. If someone sends you their certificate in e-mail, you simply have to tell your e-mail program to save it (again, handled automatically if you use Mail, and requiring one step if you use Entourage).
You can get a free certificate from a company called Thawte and can find the complete instructions (and what is probably a better explanation than the one I just provided) here. The direct page for the free e-mail certificate is here.
If you want my certificate, just ask and I’ll e-mail it to you.