The user-friendly, lightweight, and fast email client Latest Release: 3.11.0 (News), on 20 October 2014

Features

Claws Mail includes all of the features you'd expect, such as:
Multiple accounts
Threaded display
Filtering
Mime attachments
Usenet news reading and posting
SSL over POP3, SMTP, IMAP4rev1 and NNTP protocols
GnuPG support (with GPGME)
User-defined headers
Colour labels
Multiple MH folder support
Mbox import/export
External editor
Message queueing and drafting
Automated mail checking
Line-wrapping
Clickable URIs
Addressbook
Printing

More features

Claws Mail also includes a lot of useful and powerful features, only some of which are listed below:
Plugin mechanism (see below)
Actions
Templates
Built-in image viewer
Face and X-Face headers support
Reply-finder
Fast message cache system
Per-folder processing mechanism
Spell checking (with aspell)
Drag'n'drop support
POP before SMTP authentication
Automatic saving of message when composing
Handling of Unix signals
Control by command-line
Built-in GDB crash handler (just in case ;-)
Customisable toolbars
Powerful Quick Search function
'Dynamic' signatures
Font configuration
Themes mechanism (see available themes)
Automatic account selection
Message scoring
Ability to hide read messages
IMAP over SSH Tunnel
IMAP: CAPABILITY support.
Return-Receipt handling
Message priority setting
'Ignore thread' option
Anti-phishing URL check
Configurable notification of new mail
7bit and 8bit encoding of attachments
NNTP: auto-mark cross-posted messages
Configurable (non-)display of images
Full coloration of messages
User-definable quotation characters
Watch marked threads
Log window
Reply to part of an email
Replied and forwarded flags
Man page and documentation
Full GnuPG support
'On-the-fly' changing of the type of GnuPG encryption and/or signing used (MIME/ascii) in the Compose window
Manual selection of MIME type and encoding for attachments
Save all attachments in a mail at once
Save 2 or more selected files to a single file
SSL certificate management
User-definable newsgroup names abbreviation length
Address book: import from LDIF, Mutt or Pine natively
Address book: import from other mailers via scripts
Auto-completion of recipients (also via LDAP)
Harvest addresses for addressbook
Addressbook export to various formats
Per-folder properties:
Return-receipt
Save messages
Default To:
Subject simplification
Folder CHMOD
Default account
Folder colour
Offline synchronisation
Default dictionary

Plugins

The following plugins are included in the source package.
Bogofilter
Enables the scanning of incoming mail received from a POP, IMAP, or LOCAL account using Bogofilter. It can optionally delete mail identified as spam or save it to a designated folder, and also can be used to train Bogofilter.
Bogofilter is available from http://www.bogofilter.org/
Dillo HTML Viewer
Enables the viewing of html messages using the Dillo web browser, version 0.7.0 or newer. It uses Dillo's --local option by default for safe browsing. Dillo is available from http://www.dillo.org
PGP/Core
Handles core PGP functions and is a dependency of both the PGP/Inline and PGP/MIME plugins. Requires GnuPG and GPGME:
http://www.gnupg.org/gpgme.html
PGP/Inline
Handles PGP/Inline signed and/or encrypted mails. You can decrypt mails, verify signatures or sign and encrypt your own mails. Requires GnuPG and GPGME:
http://www.gnupg.org/gpgme.html
PGP/MIME
Handles PGP/MIME signed and/or encrypted mails. You can decrypt mails, verify signatures or sign and encrypt your own mails. Requires GnuPG and GPGME:
http://www.gnupg.org/gpgme.html
S/MIME
Handles S/MIME signed and/or encrypted mails. You can decrypt mails, verify signatures or sign and encrypt your own mails.
SpamAssassin
Enables the scanning of incoming mail received from a POP, IMAP, or LOCAL account using SpamAssassin. It can optionally delete mail identified as spam or save it to a designated folder, and also can be used to train a local Spamassassin (or a remote one if Spamassassin >= 3.1 is installed both locally and remotely).
SpamAssassin is available from http://spamassassin.apache.org/
Trayicon
Places an icon in the system tray that indicates whether you have any new mail. A tooltip also shows the current new, unread and total number of messages, and a contextual menu allows the most common operations.
More...
Additionally, several external plugins are available on the Plugins page. They are not included in Claws Mail to minimize the dependencies and size of the core software, but provide nice, extra functions.