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.
IMAP Folder Manager was created to manage IMAP folders for large user bases. It can list, create, delete, and rename folders, count messages in a folder, move or copy all the messages from one folder to another, and empty a folder. It supports PROXYAUTH for easy user management and has a simple but powerful argument interface, able to do things like "create folder2, move messages from folder1 to folder2, and delete folder1" in a single compact line.
imapsync is a tool for facilitating incremental recursive IMAP transfers from one mailbox to another. It is useful for mailbox migration or backup, and reduces the amount of data transferred by only copying messages that are not present on both servers. Read, unread, and deleted flags are preserved, and the process can be stopped and resumed. The original messages can optionally be deleted after a successful transfer.
rit can be used to manipulate mailboxes over IMAP, allowing users to copy and delete mailboxes and their contents. When copying a mailbox, its subscription status and all of the IMAP flags of the messages it contains are preserved on the destination server. Dynamic mailbox name translation can be performed if the mailboxes need to be renamed as they are copied.