The Bat! is the most powerful & versatile e-mail client I have ever used, and I've tried a few!
TB!; is now in version 3 which offers IMAP4 support, plain or html mail options, antispam and antivirus plugin support, a scheduler, pop mail scanner and downloader, and the unique mail ticker.
TB! won't let you open an attachment if it has a double extension so protects you from viruses which use that means to spread, and can be set to prevent opening of or issue a warning about any other attachment.
It uses its own address book format (rather than Windows address book) which is not vulnerable to being exploited by viruses.
As it does not display linked html objects it does not report back to a website or anywhere else that a mail has been received and read, e.g. by opening images linked to websites or downloading scripts or cookies.
TB! has powerful filtering and macro actions which can be used to sort mail very easily.
It includes Quick Templates to be able to add pre-written text or the contents of a file with a few simple keystrokes.
The scheduler allows reminders to be linked to mails - to trigger an alarm, start a program, send/check mail, create a mail or send a mail written earlier at a given time, or open an external document.
TB! also supports multiple mail accounts with different providers and mail servers, it can be used by more than one user so that each user has their own login and settings, and can even be used on multiple machines in client/server mode to provide network mail services.
All in all this program offers amazing value for money, and more features than you can imagine. It's easy to use whilst you learn all about its more advanced options.
To download an evaluation version:
http://www.ritlabs.comFor the most FAQ and the Macro and Filter Library:
http://www.silverstones.com/thebat/tbfaq.html