Specify in the project page that preprocess function may be ignored when dealing with several themes

When the theme choosen for htmlmail is different from the current theme, the preprocess functions of the htmlmail theme is ignored.

So it should be explain somewhere in the documentation that the preprocess functions should not be used to avoid unexpected behaviors, in order to save debugging time for developpers.

To reproduce this issue : simplely use an admin theme (drupal core feature, admin_theme module, section module or other...), select a different theme for htmlmail, with a preprocess function that exits from the script, then send a test mail from htmlmail admin page.
The sent email will not go through the preprocess function!

I try to figure an elegant patch for this, but I didn't found any.