Balsa is an e-mail client for GNOME, highly configurable and incorporating all the features you would expect in a robust mail client.

Features :

•     Support for local mailbox formats: mbox, maildir, mh
•     Allows nested mailboxes (see this screenshot)
•     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 (see the mailing list page)
•     Multiple character sets for composing and reading messages
•     Allows file attachments to outgoing messages
•     GPG/OpenPGP mail signing and encryption