http://www.ritlabs.com/en/products/thebat/
+Essential mail protocols (POP3, IMAP4rev1, SMTP) are supported with their secure layer;
+Protects your computer from viruses and worms that spread via email;
+Supports plug-in interface for anti-virus programs to avoid viruses from appearing on your system, running and
+sending infected files;
+Comprehensive Anti-spam plug-in interface to keep you protected from unsolicited commercial e-mail;
+Does not use the Windows address book and Windows-dependent HTML viewer, which have been the targets of many recent
attacks;
+Warns the user before opening suspicious attachments or even blocks them completely;
+Preview and manage mail on POP server before downloading it using Mail Dispatcher;
+Fully functional IMAP4rev1 support for on-line, off-line and combined use;
+Supports unlimited number of accounts and users;
+Gives the fully customizable message templates that save hours of typing, now with possibility to add user-defined
macros using plug-in interface;
+Powerful filtering system automates message handling;
+Scheduler notifies about upcoming events, postponed messages and performs other automated tasks;
+Built-in support of S/MIME and PGP versions from 2.6x through 8 (both clear text and PGP/MIME);
HTML mailing;
+Easily configurable user interface;
+Unique Mail Ticker? to notify about new messages;
+Multi-lingual interface supporting 15 languages "on the fly";
+Import message bases from all major email clients
-I also like how it does spell checking (on the fly). It has a free 30 day trial along with the ability to not delete messages from the server (Incase you're not comfortable with it, you can get the same messages with your old client as well.