OS X Email Client

[Brock Woolf]

Hi,
I currently use Apple Mail for my email client and I do like the 
integration of the program with the Address Book, but I find it to be 
limited in it's functionality. I would like to know if anybody knows of 
a better email client than Mail that you use on your Mac.
P.S. Please don't mention Outlook, because I do not trust Microsoft 
products.

Thanks,
Brock