Claws Mail is an open source email and news client written in C (with GTK+) distributed under the GPL. Claws mail is a mail client formerly called sylpheed-claws.
Claws Mail is a lightweight email client application suitable for Linux-based operating systems that use a lightweight window manager/desktop environment. Claws Mail runs on BSD, Linux, Mac OS, Solaris, Unix, Windows.
Claws Mail offers features like storing mail in the MH mailbox format via a plugin, extensible using loadable plugins, other storage formats, feed reader, calendar management, mail filtering, desktop integration features such as system notifications for new emails, offline email management, and integrated encryption support, Search and filtering, Security (GPG, SSL, anti-phishing), Import/export from standard formats, Face X-Face support, can be extended with plugins, such as Anti-spam (SpamAssassin, Bogofilter), RSS aggregator, HTML viewers (Dillo, Gtkhtml2, Fancy (WebKit)), Trayicon, Laptop mail LED handler.