OpenPGP is the most widely-used e-mail encryption standard in the world. Originally derived from PGP (Pretty Good Privacy), first created by Phil Zimmermann in 1991, the standard is now defined by the IETF in RFC 4880.
This module provides encryption for Drupal's outgoing e-mails using GNU Privacy Guard, an open-source implementation of the OpenPGP standard.
The project isn't maintained anymore, since I lack both the necessary time and belief in the module's usefulness. Anyone disagreeing on the latter part may take over the project, if they want.