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.