Heirloom mailx (formerly known as "nail") is derived from Berkeley Mail and provides the functionality of the System V and POSIX mailx commands. Additional features include support for MIME, IMAP (including caching and disconnected use), POP3, SMTP, S/MIME, international character sets, maildir folders, message threading, powerful search methods, scoring, and a Bayesian junk mail filter. Mailx can be used as a mail batch language in nearly the same way as it is used interactively. It can thus act as a mailbox filter, can fetch mail from remote accounts, and can send files as attachments.
Netbiff is an advanced version of the traditional biff (a mail-checking program). Features include BSD mailbox and IMAP4 support, a backend plug-in system (for other protocols), arbitrary numbers of backends, pluggable interfaces (GTK and text-based), and arbitrary actions on biff.
p3c is a pop3 checker allowing you to inspect your mailbox before downloading and to delete any unwanted messages. For you to choose wich messages to delete, p3c will show you the "To:", "From:" and "Subject:" portions of the mail and then ask you if you want to delete it or not. p3c works from the console using a simple text interface.
Perdition is a fully featured POP3 and IMAP4 proxy server. It is able to handle both SSL and non-SSL connections and redirect users to a real-server based on a database lookup. Perdition supports modular based database access. The distribution ships with modules for ODBC, MySQL, PostgreSQL, GDBM, POSIX Regular Expression, and NIS. The API for modules is open, allowing abitary modules to be written to allow access to any data store. Perdition can be used to create large mail systems where an end-user's mailbox may be stored on one of several hosts, to integrate different mail systems together, to migrate between different email infrastructures, and to bridge plain-text, SSL, and TLS services. It can also be used as part of a firewall.
pm2imap.pl is a perl script that will convert Pegasus Mail 3+ mailfolders (and trays) to Unix mbox format in the same Folder structure as your tray structure. This allows you to read your mail via IMAP instead of locally via the filesystem, which allows for "remote" reading of your mail.