GNUMail.app is a clone of NeXT/Apple's excellent Mail.app application.
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.

  Features include:
    * 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;
    * A full MIME types manager;
    * Address Book with groups support;
    * Find Panel with regular expressions support;
    * Automatic or manual fetching of your mails (user's choice);
    * Powerful filters that supports regular expressions;