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.
Hep Message Server is software that transfers messages between different messaging systems. Eventually, it should transparently route messages between email, weblogs, and instant messaging. Currently Hep does one thing: fetches news feeds from Web sites and lets you view them in your email client.
Twisted Mail provides client and server implementations of SMTP, POP3, and IMAP4. These differentiate themselves from the Python standard library implementations both by presenting a much higher-level, easy-to-use interface and in their server components, which allow the implementation of custom servers for each protocol without dealing with protocol-level issues. Twisted Mail includes a simple demonstration email server which accepts messages over SMTP, stores them in a Maildir arrangement, and can serve them to clients over POP3.