PMDF System Manager's Guide

When PMDF is sending mail, it can get into a loop sending mail back to itself if the destination domain has an MX record of 0.0.0.0 or 127.0.0.1. The loopcheck keyword causes PMDF to use the SMTP XLOOP line on the EHLO response so that PMDF can detect that it is sending mail back to itself, thus preventing a loop.