Hello All: I have an odd problem where some sites are refusing to accept mail from me. 99% of sites will accept mail from me, a but a few reject mail with the following response: Feb 3 20:33:04 ringo postfix/smtp[7229]: B83C519B01: to=<person [ at ] destination [ dot ] COM>, relay=smtp.destination.COM[192.168.1.10], delay=7507, status=deferred (host smtp.destination.COM[192.168.1.10] said: 450 <person [ at ] walma [ dot ] org>: Sender address rejected: Domain not found (in reply to RCPT TO command)) (I've changed email and IP addresses to protect the innocent.) I assumed that the problem was DNS, but I've verified that my DNS is RFC compliant (or so www.dnsreport.com says it is), and besides, the sites to which I cannot send generally (with one exception) have no problem sending mail to me. I have a fixed IP, I don't appear to be on any blacklists and my IP is properly delegated to me, that is, reverse DNS works. The problem sites tend to be large ones, government departments and a large corporation. They tend not to be too quick to respond to my requests for helping in debugging my problem. I am running Postfix 2.1.5-r1 and BIND 9.2.5. Any thoughts on what I should look at? Thanks in advance for any help. Michael