Dr. Wietse, Thank you again for your time. I have been an off-and-on-again user of Postfix for many years, and it is a real honor (and surprise!) to get support directly from its originator. > The hop-count limit is reached in the REMOTE mail server. I suspected as much, but I wasn't sure. Thanks for clarifying this. > There are two possibilities: > > - The failing message already has lots of Received: headers (which > is something that you may be able to fix by removing some or all). It doesn't because the message is being created at the server where Postfix is installed. > - The REMOTE mail server has an unreasonably-low hop-count limit > (which is something that only the remote system adminstrator can > fix).
This is the only possibility that remains... and since the system administrator for that system is difficult to communicate with, my guess was that the single innocent "Received:" header added by Postfix was the tipping point. For this reason, I used the "header_checks" parameter to strip it off before sending it to the external mail server... ...and now everything works. Thank you very much! Your work makes a real difference. Ahmed.