Hello,
Out Exchange 2010 server has recently been rejecting a lot of inbound mail, and the senders are receiving NDR's with the following:
554 5.7.1 This message has been blocked because the return email domain is invalid.(failed to obtain DNS record for domain sender.tld)
This does not happen consistently, someone that gets blocked will be able to successfully re-send the message say 10 minutes later. Our server is a Hub, CAS & MBX, there is no Edge server in our organization and the anti-spam agents have not been
added to the hub role. I've already run Set-SenderReputationConfig -Enabled $false, and we are still bouncing incoming messages.
I've looked at some of the senders, and the name of the sending mail server does exist in DNS, and that IP does have a PRT with the same name. Is there a way to completely disable/skip reverse lookups?
Thanks,
Joe
