The Pretty Good Privacy (PGP) email encryption system was developed by Phil Zimmerman. For encrypting messages, it actually uses AES with up to 256-bit keys, CAST, TripleDES, IDEA and Twofish. RSA is also used in PGP, but only for symmetric key exchange and for digital signatures, but not for encryption.
Source: KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of Computer Security,
John Wiley & Sons, 2001, Chapter 4: Cryptography (pages 154, 169). More info on PGP can be found on their site at http://www.pgp.com/display.php?pageID=29.