Why isn't the default template used? I set Mime Mail as the sitewide default mailing system and overrode mimemail-message.tpl.php. My custom template is used correctly for e-mails like account activation, but when I send an HTML e-mail with Rules, the template isn't loaded: I simply get an unstyled HTML text.

Did I forget something (e.g. a configuration for Rules)?
Is your administration theme the active theme when the Rule fires? This documentation page about using a custom mail template with Mime Mail might be referencing the issue:


  If you use sub-themes or multiple themes, especially if you use a different administration theme than your default theme, you might end up with a different theme to what you expect. 


In the Mail System module configuration settings (admin/config/system/mailsystem), try changing the Theme to render the emails from Current to the Default theme or to the theme where your overridden mimemail-message.tpl.php lives.