How do I send plaintext email?
I've spent some time trying to achieve the same goal.
I didn't find a lot of documentation on MimeMail module, I inspected the code and found a solution :
  $message = drupal_mail('my_module', 'my_module_key', 'email@mail.com', $language,
    $params = array(
      'subject' =&gt; $subject,
      'plain' =&gt; TRUE,
      'plaintext' =&gt; $mailBody,
    ));

You just need to use 'plain' =&gt; TRUE and put your mail content in 'plaintext' =&gt; $mailBody