SSL/TLS security in emails

I know about the use of SSL(Secure Socket Layer) for securing web sites. But in email clients also, there is a option of using SSL e.g Netscape client has a option to enable SSL for outgoing messages. How is this option implemented for email clients. I believe email client uses a smtp server for sending messages. How and where SSL comes in picture for emails?
My 2nd part of the question is TLS(Transport Layer Security) security for emails. Netscape Client has a option to activate TLS also. Howis this implemented ? Netscape email client provides 3 options to use SSL/TLS i.e “never”, “if possible” or “always”

regards
-mukul