Things I love about Sylpheed:

-being able to switch from one identity to another (work email, home email, etc) very quickly and easily
-no HTML in sight! Yay! I hate HTML mail.
-ability to add custom headers, or not.
-the fact that Sylpheed doesn't insist on including a mail header saying what program was used to create the email. (Unless I enable custom headers. Rats that Sylpeed does identify itself then.)
-ability to use PGP with the program
-ability to download program in .zip format, unzip to a folder, and then use immediately - Windows installer available to those who need it, but not forced upon all. Yay!
-program works when all .dll files are in Sylpheed folder along with .exe file. No need to clutter Windows registry
-signature files stored as plain text within Sylpheed folder.
-ruler to show how many characters in a line
-ability to let text wrap, or click one button and have fixed-length lines of an appropriate length
-ability to change character encoding. And one of the encoding schemes to choose from is Unicode. Yay!
-Sylpheed is available for Linux and Windows. Very handy, as I'm now using a Windows machine but plan to move to Linux soon.
-program released under GNU General Public License. Most reassuring.

I've only just started using this program, but already like it better than TheBat, Eudora, Outlook, etc. It's the best GUI-based email program I've come across so far on the Windows platform. When it comes to things I like in an email program, the only thing that rivals Sylpheed is Pine, and that's a whole different kind of beast anyway.

Thank you for this excellent program!