muttdown is a sendmail-replacement designed for use with the mutt email client which will transparently compile annotated text/plain mail into text/html using the Markdown standard. It will recursively walk the MIME tree and compile any text/plain or text/markdown part which begins with the sigil "!m" into Markdown, which it will insert alongside the original in a multipart/alternative container. . It's also smart enough not to break multipart/signed
Premail adds support for encrypted e-mail to your mailer, using plain PGP, PGP/MIME, MOSS, or S/MIME. In addition, premail provides a seamless, transparent interface to the anonymous remailers, including full support for Mixmaster remailers and the nymservers. Nymservers provide cryptographically protected, fully anonymous accounts for both sending and receiving e-mail.
qmHandle is a tool which can be used to manage the qmail message queue. It has more features than qmail-qread and qmail-qstat.
X-PGP-Sig header utility for Emacs. It can insert/verify PGP signatures in the X-PGP-Sig: mail/news header. x-pgp-sig.el is for MH(mh-e, Mew and so on), Gnus, Semi-gnus, wl(Wanderlust) VM, cmail etc... (MUA on Emacs). . English documentation is not available, only Japanese.