The user-friendly, lightweight, and fast email client Latest Release: 3.10.1 (News), on 9 June 2014

The GPG plugin

The GPG plugins (PGP/Core, PGP/Mime and PGP/Inline) provide digital signature and encryption features: Every account can be configured with different defaults regarding signature and encryption, making it easy to quickly switch between sets of parameters.
Although deprecated, the PGP/Inline method of signing or encrypting emails is still available, because some mail user agents like Outlook do not handle PGP/Mime well, more than ten years after creation of this standard.

GnuPG and GPGME are required, http://www.gnupg.org/

Here are a few screenshots illustrating the features: