I recommend
Becky.
My history of e-mail includes pine on VAX/VMS terminals, and later Slackware,
followed by Mutt in Debian for a very long time. Eventually, I started using
Windows more for my workstation (ca. 1999). Outlook Express and Thunderbird
are not enough for me, but I could recommend them to family and friends.
I too send and prefer to receive mail in text format. As you will notice I even
post in forums with consideration for reader-friendly line length out of habit.
I used The Bat! for a couple years and wrote a few plugins, but it is frail software.
Eventually I was considering writing my own client or a Mutt port to Windows.
I gave Becky another try, and agreed with the elegance I considered spartan
when evaluating it some years earlier (before choosing The Bat (unfortunately)).
I have been using Becky for a couple years now and written a couple plugins.
I really like it. It benefits from the direction of a single intelligent developer.
It provides everything I want, and a spectacular plugin API to make adding
most other things trivial. There are many plugins and an active community.
http://www.rimarts.co.jp/becky.htm
It will work forever without registration or payment, but as a developer I
prefer to pay people for their hard work. I think others should too.
I use qmail and tmda on my mail server, notable as my means of spam
protection is decoupled from my mail client. I also use a desktop mail
delivery relay to transform outgoing mail for my own advanced needs.
P.S. Some readers may jump to obscene and offensive conclusions about my use
of tagged message delivery and mailing lists. Those conclusions are very wrong.