This issue just crept up in trying to send to an internet/external domain. Using an Exchange 2007 server this suddenly appeared with only sending to one specific domain. Receiving mail is no issue and sending to other domains does not generate this issue. Only one domain is causing this issue: #554 5.4.4 SMTPSEND.DNS.MxLoopback; DNS records for this domain are configured in a loop ##.
Is this an issue on my end, since I have had this server in place for many years and never had this problem, or is it on the recipient side?