I thought immediately of enigmail
http://enigmail.mozdev.org/home/index.php
It allows you to encrypt/sign email using the mozilla email clients, which can send/receive mail from your yahoo, gmail, hotmail.
Your title mentions SSL (openssl), but that's another sort of thing. It's for creating secure connections (to your email server or any other), not for sending encrypted email or digital signing.
Also, both the sender and receiver will have to be signing/encrypting, otherwise it's pointless.