Users have reported that intermittently, inbound email is being returned to sender with an invalid domain name: #554 5.4.4 SMTPSEND.DNS.
I'm seeing DNS lookup errors in both our TransportRoles > Logs > Mailbox > Connectivity > Delivery and Submission folders.
In Delivery:
2014-03-12T19:24:53.739Z,08D102BA4526E381,SMTP,mailboxtransportsubmissioninternalproxy,+,Undefined 00000000-0000-0000-0000-000000000000;QueueLength=0
2014-03-12T19:24:54.364Z,08D102BA4526E381,SMTP,mailboxtransportsubmissioninternalproxy,>,Non-existent domain reported by 192.168.1.1. [Domain:Result] = (our mail server name):InfoDomainNonexistent;
2014-03-12T19:24:54.364Z,08D102BA4526E381,SMTP,mailboxtransportsubmissioninternalproxy,-,Messages: 0 Bytes: 0 (The domain name does not exist. Please correct the address and try again.)
2014-03-12T19:24:54.364Z,08D102BA4526E380,MapiDelivery,Mailbox Database 0020632088,-,Messages: 1 Bytes: 21454 Recipients: 1
These messages are repeated every 65 seconds for a 15 minute period and then stop.
In Submission we see many more like this:
014-03-12T08:40:36.445Z,08D102BA3F4FE575,SMTP,mailboxtransportsubmissioninternalproxy,+,Undefined 00000000-0000-0000-0000-000000000000;QueueLength=0
2014-03-12T08:40:36.445Z,08D102BA3F4FE575,SMTP,mailboxtransportsubmissioninternalproxy,>,Non-existent domain reported by 192.168.1.1. [Domain:Result] = (our domain name):InfoDomainNonexistent;
2014-03-12T08:40:36.445Z,08D102BA3F4FE575,SMTP,mailboxtransportsubmissioninternalproxy,-,Messages: 0 Bytes: 0 (The domain name does not exist. Please correct the address and try again.)
These messages repeat every 120 seconds many times throughout the day.
My questions:
(1) If we see the messages in the Mailbox Connectivity Submission log, does that mean that emails we are sending may not be getting to their recipients? (We are not receiving any DNR reports ourselves, just some people sending messages to us, and it's not consistent, i.e., they can resend and we do receive their second message.)
(2) Do these messages appear once for each message that fails, or do they just report in these fixed number of seconds intervals, and that any emails that are attempted to be sent or received during that interval fail? (There seem to be more intended recipients who are getting DNR reports from us than there are log messages in the Mailbox Delivery logs, and the times don't seem to line up well.)
We have ECP configured for Internal and External DNS lookups to our local host first and then our external DNS servers provided by our ISP through our firewall. I've read that for Internal DNS lookups, we shouldn't look to an external DNS server, might that explain this somewhat random occurrence?