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Now the property of Rose City Software, Calypso was originally developed by Micro Computer Systems. While the version tested here is freeware, the next version -- Courier 3.5 is Shareware (you can purchase Courier for about AU$40 if 3.3 is your choice).
The user interface is reasonably attractive but nothing exotic. The idea was to produce an e-mail client with all the things you really need without a bunch of unnecessary bells and whistles. Calypso's mailbox format is purposely different to the various standard formats in order to offer protection from worms. Unfortunately, this also means you will need to transfer any addresses from your existing e-mail client to Calypso manually.
Calypso still comes with handy e-mail features that we all know and love, such as auto-response messages, message templates, and spell check -- the latter supports multiple languages. The toolbar can be customised, and Calypso also offers mail filtering, optional HTML support, and address auto-completion. PGP encryption support is available, but it only runs on Microsoft Windows, and requires 6MB of disk space.