code: function setReplyToAddress($mailingId, $auto = true, $customEmail = null)
      {
          $queryParameters = array(
              'auto' => ($auto == TRUE) ? "true" : "false",
              'customEmail' => $customEmail
          );
  
          return $this->post('mailings/' . $mailingId . '/settings/replyto', null, $queryParameters);
      }
docs: Sets the reply-to address of the mailing identified by the given ID.
  
  @param string $mailingId
  the ID of the mailing
  @param bool $auto (default = true)
  If true, the Maileon autorecognition will be used and emails will be saved within Maileon. If false, a custom email address can be passed which gets all mails forwarded.
  @param string $customEmail (default = empty)
  If $auto is false, this email will be used for manual responses.
  @return
  @throws MaileonException