Wikipedia
GPGMail
GPGMail is an extension for Apple Mail that provides public key e-mail encryption and signing. GPGMail works under Mac OS and the actual cryptographic functionality is handled by GNU Privacy Guard.
GPGMail was first released on Feb 7, 2001, by Stéphane Corthésy, and since 2010 maintained by the GPGTools Team. Both GPGMail and GNU Privacy Guard are free open-source software.