Thunderbird

Thunderbird is the new Mozilla project Mail and News reader, now sporting a spiffy three-pane interface that (like Outlook 2003) supports a vertical preview pane.

A neat aspect: customizing toolbars works just like in Mac OS X:

• Address Book-enabled builds
• Enigmail - PGP/GPG support
• SPF extension - a mixed XUL/CGI solution to check incoming e-mail using SPF.