mujMail is a powerful and graphically atractive email client for mobile devices. Its operations are multithreaded, so you can send mail, retrieve mail from multiple mailboxes, and change settings simultaneously. It uses RMS to store mail messages persistently on the device, so don't have to connect to a server to read old mail. It has the unique ability to redownload a damaged message or body part and repair it.
Usermin For Webmail is a modified version of the Usermin package that is pre-configured for use as a Web-based email client. It is designed to work on systems running a local IMAP server, such as Dovecot. By default, users can read email, setup autoresponders and forwarding with Procmail, filter mail with simple rules, change their passwords, and manage folders.
Inverse SOGo Connector for Thunderbird is an extension which transforms Thunderbird into a full DAV client for groupware servers such as SOGo, OpenGroupware.org, and Citadel. It does this by adding support for remote DAV address books and by adding features to be used along with the Lightning calendar extension. It is a merge of the Lightning Enhancer and the GroupDAV Addressbook Connector.
libSmtpThread is a multi-threaded C++ library for sending mail via SMTP. The library provides a connection pool that initializes a set of threaded SMTP connections. Emails can be pushed into the pool, which accepts all mail and delivers them to a free connection. The delivery to the SMTP server is processed in a separate thread, so you don't have to wait for mail to be accepted. It has been successfully tested with Postfix.
checkoutlook.pl is a Perl script to check mail on Microsoft's Outlook Web-based mail reader. The script currently only checks for new mail and then logs out, printing the sender and subject of the message, if applicable. The script can be run in daemon mode, in which case it runs in silence until a message is received.