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
• Stampar
• 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