GNUMail.app for Mac

GNUMail.app is a fully featured mail application running on Linux (or FreeBSD, OpenBSD, etc) and Apple Mac OS X. It uses the GNUstep development framework or Apple Cocoa, which is based on the OpenStep specification provided by NeXT, Inc. GNUMail.app is licensed under the GPL, and the full sources of the application are available. GNUMail.app was written from scratch. It uses Pantomime as its mail handling framework.

Version 1.1.0 has the following supported features: Multiple POP3 (with APOP support) accounts with local cache support (for leaving messages on server); Multiple delivery agents (SMTP or local mailer); Mail spool file support for receiving; Good IMAP support with local caching; Full RFC822 and RFC2822 support; Read, compose, bounce, reply and forward mails; Mailbox management (create, delete, locking and transfer messages) using the Berkeley mbox format; Personal preferences management; MIME support (decode and encode); A full MIME types manager; Quoted Printable and BASE64 support; Address Book with groups support; Find Panel with regular expressions support; Automatic or manual fetching of your mails (user's choice); Powerful filters that support regular expressions; GPG (PGP) support through a bundle; Messages threading.