When ManyMail MIME is enabled, it would make sense to adjust some fields to become text_format.
That way the WYSIWYG module can hook into it and facilitate the creation of HTML e-mail.

The following questions arise though:

  • On which fields do we want it?
  • Where do we want it: front, admin or both?
  • Should this be a hook_form_alter in ManyMail MIME or part of ManyMail Core?

Postponing this issue until I have some clear answers and feedback on this.