getmail is intended as a simple, secure, and reliable replacement for fetchmail. It retrieves email (either all messages, or only unread messages) from one or more POP3, SPDS, or IMAP4 servers (with or without SSL) for one or more email accounts, and reliably delivers into qmail-style Maildirs, mboxrd files, or through external MDAs (command deliveries) specified on a per-account basis. getmail also has excellent support for domain (multidrop) mailboxes, including delivering messages to different users or destinations based on the envelope recipient address.
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.
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.
XCmail is a MIME and multi POP3 capable mailtool for X11 using the Xclasses X11 layout library. Its main purpose is to read and write mail, allowing any kind of data to be attached. It can handle MIME types, has helpers to show different types, and can encode and decode binary data into ASCII to allow it to be transported as mail over the Internet. It has many built-in functions such as 2 vCards, unlimited POP3 servers, SMTP, SMTP after POP3, SMTP-AUTH (RFC2554), addressbook, encoders, filters, a special spam filter, spell checker, xface, and a new "virtual folder" concept. It also uses external programs to enhance its feature set. XCmail can be extended through a plugin interface.
Designed as a Perl framework to support many different types of POP and email servers, Mobile POP Relay is useful for mobile users. Once a user successfully authenticates against a POP server, relaying is allowed through the server for a specified time period. It supports Ipop3d, Qpopper, and Vpopd with Sendmail or Exim.
Eremove is a small GTK-based application with a simple interface. It allows you to view a summary of the contents of a mailbox on a POP3 server. It displays a list with size, atachment, from, date, and subject. It allows you to preview the headers/contents of each email, and select some or all of the messages for deletion.