Optional setting "Use Return-Path parameter"

I'm pretty sure that almost all mail servers support this additional parameter.

But I've got errors on particular corporate mail server. Messages simply were not sent if custom Return-Path parameter had been set.

That's why I decided to provide additional config for HTML Mail module. Maybe that can help somebody else with the same problem

Sorry for being stubborn. Return-Path is not sent in headers, but in additional parameters.

Let's look at Drupal Core code

  $default_from = variable_get('site_mail', ini_get('sendmail_from'));

...

  if ($default_from) {
    // To prevent e-mail from looking like spam, the addresses in the Sender and
    // Return-Path headers should have a domain authorized to use the originating
    // SMTP server.
    $headers['From'] = $headers['Sender'] = $headers['Return-Path'] = $default_from;
  }

So the Return-Path header is set.
Later in DefaultMailSystem class (and in HTMLMailSystem too) Return-Path is moved from headers to additional parameters

    // If 'Return-Path' isn't already set in php.ini, we pass it separately
    // as an additional parameter instead of in the header.
    // However, if PHP's 'safe_mode' is on, this is not allowed.
    if (isset($message['headers']['Return-Path']) && !ini_get('safe_mode')) {
      $return_path_set = strpos(ini_get('sendmail_path'), ' -f');
      if (!$return_path_set) {
        $message['Return-Path'] = $message['headers']['Return-Path'];
        unset($message['headers']['Return-Path']);
      }
    }

      if (isset($message['Return-Path']) && !ini_get('safe_mode')) {
        // On most non-Windows systems, the "-f" option to the sendmail command
        // is used to set the Return-Path. There is no space between -f and
        // the value of the return path.
        $mail_result = @mail(
          $message['to'],
          $mail_subject,
          $mail_body,
          $mail_headers,
          '-f' . $message['Return-Path']
        );
      }

In htmlmail.mail.inc we have similar code

Of course to set that parameter, mail sending user needs to be a trusted user for mail system
I tried to contact system administrator to do that, but unsuccessfully
And I had to fix that problem ASAP.
So I decided to provide possibility not to add additional Return-Path
This situation happens rather hardly, but I think that additional option is helpful sometimes

Return-Path is set by Drupal Core, not a custom module
Look at includes/mail.inc, drupal_mail()

  if ($default_from) {
    // To prevent e-mail from looking like spam, the addresses in the Sender and
    // Return-Path headers should have a domain authorized to use the originating
    // SMTP server.
    $headers['From'] = $headers['Sender'] = $headers['Return-Path'] = $default_from;
  }

Well, I can cut down this param with a hook of course. As you wish.