Quaik is a lightweight mailer for single or bulk mailing. Supports Address Book, HTML, SSL.
Queue Sync is a Palm Conduit for MicroSoft Entourage.
Revolver Mail combines all the elements of a personal information manager - an email client, an address book, and a calendar - in a single, integrated, user-friendly interface. Commercial versions are also available.
SecExMail provides email encryption software that uses RSA public key encryption and the Twofish block cipher. It features PGP style email keys, but without the dependence on email software-specific plugins.
Send As Email CMM
Send As Email CMM lets you send files as email attachment via context menu.
(shareware) Serial Mail is a plug in for Apple Mail to generate serial mailings from a template using data provided by an Address Book group. The program will process the message body of the (selected) template message and replace strings like and by their corresponding value from the Address Book for each recipient.
Signature Randomizer allows you to select a different signature for your favorite eMail application each time you login or boot your computer and optionally after a customizable time-interval.
SignatureProfiler is a plugin for Apple's Mail application that adds missing features from the way that signatures are handled and managed with your outgoing mail messages, especially if you have multiple mail accounts.
(commercial) SpamSieve gives you back your inbox by bringing powerful Bayesian spam filtering to Mac e-mail clients. It's quick and easy to control SpamSieve from within your mail client, and you can customize how it interacts with the rest of your message sorting rules.
spamtrainer was written to assist mail services administrators in updating and maintaining their SpamAssassin bayes database. When run, it will read the designated HAM and SPAM mailboxes, update the SpamAssassin databases and delete mail that has been learned from.
SpeedMail is a speedup Mail.app.
SweetMail is an e-mail program that supports multiple accounts and threading.
(shareware) ThinkArch allows you to archive MS Entourage email messages in DEVONThink Pro.
UTF-8 Output support for Pine
A small patch to enable UTF-8 output in Pine.
Webmailer is a program (actually a preference pane) for Mac OS X which lets you set it to open any URL or run any shell script, complete with the parameters lifted from the mailto link.