In article <9EC12CC9-81CD-4C3E-ADD1-283615767B17@xxxxxxxxxxx> you write: >On Mar 30, 2014, at 6:44 PM, John C Klensin <john-ietf@xxxxxxx> wrote: >> Whether one accepts it or not, "been here already" tests are not >> about debugging. They are about loop detection and DoS attacks. > >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. R's, John