Hi James, in the past I found Thunderbird & Evolution were not satisfying my needs. Eventually I found Claws Mail http://www.claws-mail.org/ and I have been extremely happy with the way it handles mail. It is plain text only, images can be displayed inline (at the end of the email) and set-up, filtering etc.. is very easy. Maybe Claws Mail will suit your purposes.
cheers, Meryl -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html