Change mailsystem used on webforms on a per email basis
I think you can do this via hook_mail_alter you'll need to write your own implementation but should be possible.

From what I can see in webform.submissions.inc inside webform_submission_send_mail()

// Build the e-mail headers.
    $email['headers'] = theme(array('webform_mail_headers_' . $node-&gt;nid, 'webform_mail_headers'), array('node' =&gt; $node, 'submission' =&gt; $submission, 'email' =&gt; $email));


It should be possible to use hook_mail_alter to choose the right submission and also the e-mail address it's being sent to.