MSN Explorer Tech support states "that MSN Explorer and MSN.com send web-based HTML e-mail ONLY and cannot be configured to send plain text."
However, if you use MSN as your mail service (i.e., email@example.com) you can simply go to Hotmail.com and send e-mail from there in plain text (via their so-called PASSPORT system). Same inbox, address book, etc.
You just have to remember to send or reply to POP3 and plain text e-mail from Hotmail and not from MSN. A real nuisance but at least an option.
Unfortunately the transfer to Hotmail from MSN e-mail takes you through 3 screens and requires you to reenter your password even if you checked the "remember my password" box.
I'm currently on a mailing list where one of the members uses MSN for email and yes, it is very annoying to get email from that person in all HTML (when you are using an email client that doesn't support HTML and no, getting a new program is not the answer! (Anymore than forcing Microsoft Windows users to switch to something more sane, like Unix) It's nice to know there is a way that person can avoid sending HTMLized email, but somehow, given the instructions (and the @#$&@# attitude of Microsoft towards Internet standards and conventions) that this person will change.