I recently had to edit sendmail to use office365 as my mail relay. Due to the requirements of Office365 I had to set the return path and replyto parameters in the phpmailer class.
The following lines of code need to be added after lines #95 and #126 in the wp_mail_smtp.php file.
$phpmailer->ReturnPath = $phpmailer->FromName;
$phpmailer->AddReplyTo($phpmailer->FromName, $phpmailer->FromName);
It would have been nice to have an extra field in the options to set the replyto address manually. Perhaps an update?