Pretty Good Privacy
Are you worried about someone reading your emails? Of course you know that writing an email is similar to writing a message on a postcard. Anyone can read the contents at any time. In fact, it's even worse than that, because emails get copied and stored along the entire path from where it is sent to where it is received. Under normal conditions, there could be dozens of copies of your email all over the net at any time.
There is an answer and it's called Pretty Good Privacy. You can purchase a copy if you want with all of the bells and whistles, or you can simply download the freeware copy (see references below).
What you do when you want secure email is install PGP on your system. Then you must sign up for an encryption key with Verisign.
The procedure for getting a certificate is a little complicated so I suggest that you visit the appropriate web sites for more information.
Once you own a digital certificate (which allows you to prove you are who you say you are) you can go ahead and install it in PGP. Once that has been accomplished, you are ready to send and receive encrypted emails.
One thing to remember: this is really a pain. It's nice to be able to encrypt emails, but it's a lot of work and you have to maintain it. For example, if you want your sister to be able to read your encrypted emails, you have to give her a copy of the key (on floppy disk or in the mail). So my recommendation is to only use this when necessary.
When I thought it through, I found that with 99% of the emails that I send I don't care who reads them. There is nothing personal or particularly important. PGP serves fine for the remainder.