Often client php unsecure web scripts get abused by spammers and tracking which vhost is responsible for spam from maillog is simply impossible if there is no identificator for each vhost php mail() function. Here is how to add it to iworx vhost skeleton (or to vhost conf file).
I’m not sure if one can force iworx to update all vhost conf-s from templates - then probably you would have to write shell script that adds this line to every vhost file if not alreay there. Shouldn’t be too hard.