Modules, add-ons, and plug-ins for mail clients make easy the GPG / PGP encryption and/or signature. 

See also : Secure email-clients with PGP/MIME.
 
 

WINDOWS
 

Outlook Express	GnuPG	(included in WinPT 1.0)
Eudora	GnuPG	(included in WinPT 1.0)
Outlook	GnuPG	G-Data (free software)
Mozilla - Netscape 7	GnuPG	Enigmail (free software)
Thunderbird Mail	GnuPG	Enigmail (free software)
ICQ (Miranda)	GnuPG	GPG plugin (free software)
ICQ	PGP 6.5	PGP-ICQ (freeware)
Pegasus Mail	GnuPG	QDGPG (free software)
Becky!	GnuPG	BkGnuPG (freeware)

LINUX
 

KMail (KDE)	GnuPG - PGP 2 / 5	(included in KMail)
Mozilla - Netscape 7	GnuPG	Enigmail (free software)
Thunderbird Mail	GnuPG	Enigmail (free software)
Evolution	GnuPG	(included in Evolution)
Pine	GnuPG - PGP 2 / 5	pgp4pine / pgpenvelope (free)
Gnus/Emacs	GnuPG - PGP 2 / 5	Mailcrypt (free software)
Mutt	GnuPG - PGP 2 / 5	Mutt (free software)

MAC OS X
 

Apple Mail	GnuPG	GPGMail
Entourage	GnuPG	EntourageGPG (free software)
Eudora	GnuPG	EudoraGPG (free software)
Mozilla - Netscape 7	GnuPG	Enigmail (free software)
Thunderbird Mail	GnuPG	Enigmail (free software)
Mailsmith	GnuPG	Mailsmith-GPG