>>Are there MTAs that make use of the Received: header for loop detection? > >Mine has since 1998. If the SMTP daemon sees 100 Received: headers in >an incoming message, it rejects it. This is both in qmail's SMTP >daemon and mailfront, a better SMTP daemon often used with qmail. A little more poking around finds that sendmail and postfix also do, although their default limits are 25 and 50 headers, respectively, rather than 100.