Balsa is a mail reader for the GNOME Desktop.  It supports many features:

* Support for local mailbox formats: mbox, maildir, mh
* Allows nested mailboxes
* Support for POP3 and IMAP mail access protocols
* Printing
* Spell Checking
* Multi-threaded mail retrevial (optional)
* MIME support (view images inline, save parts)
* Supports SMTP and/or use of local MTA, ie. Sendmail
* Address Book that integrates with GnomeCard
* Highly configurable
* Active, open development
* Multiple character sets for composing and reading messages
* Allows file attachments to outgoing messages
* GPG/OpenPGP mail signing and encryption

WWW: http://pawsa.fedorapeople.org/balsa/