WP PGP email allows you to add a PGP key to a user profile, and then encrypts any email sent to that user.
Based on a Gist by Tim Nash: https://gist.github.com/timnashcouk/dd1ff318489f6bfd8743
Uses php-gpg by Jason Hinkle: https://github.com/jasonhinkle/php-gpg
Requires PHP 5.3 or later.