Last time I ran evolution it was a dog, and very buggy. I have found thunderbird to be very responsive and fairly stable, it's also nice to have the exact same email and browser on windows and linux, since I switch back and forth alot. Evolution doesn't run on windows afaik. OTOH, evolution is highly integrated with a PIM as well.
regarding the backend mail daemons, I haven't done much with sendmail recently, I'm currently using exim for my mailservers..
Do you really think that the MTA needs to be customized, or just the rewriting rules?
Bill