PGP has become a worldwide de facto standard for E-mail encryption. PGP is well featured and fast, and has excellent user documentation.
Source code is free.
The Massachusetts Institute of Technology is the distributor of PGP version 2.6, for distribution in the USA only. It is available from net-dist.mit.edu , a controlled FTP site that has restrictions and limitations, similar to those used by RSA Data Security, Inc., to comply with export control requirements. And when doing a kermit download to your PC, specify 8-bit binary mode at both ends. There are two compressed archive files in the standard release, with the file name derived from the release version number. If you don't have any local BBS phone numbers handy, here is a BBS you might try.