It sounds to me like you are saying you need to specify the font (not character set) used to render text-based email.

I'm fairly sure you can't do that. It is the responsibility of the mail reader to know that text-based emails need to be rendered in fixed-width fonts. My personal mail reader (evolution) does this correctly.

Maybe it is possible to configure your mail reader to not show text-email in a fixed width font, and maybe some mail readers are broken out-of-the-box ... but still, I don't think this is your problem.