0

We have a strange problem. Our Exchange (old guy Exchange 2010) server doesn't deliver mail to certain address. When checking I see that the domain of recipient has 2 MX records

  • MX 10 good.server
  • MX 100 bad.server

Unclear why but our Exchange try to deliver mail only to the second server. And fails as the second server is bad. 2 questions are arising

  1. Theoretical one - why?
  2. Practical one (if the theoretical one remains unanswered) - what is the way to force Exchange to deliver mail to the good.server?

Update: I use for the test centralops.net/co . I see there two MX records:

  • 10 somedomain.mail.protection.outlook.com
  • 100 relay.rzone.de

And the test mail passes all checks and is going through the first MX. In the Toolbox of Exchange I can find a kind of trace and I see there that Exchange try to send the mail to the second server and after many attempts get status FAILED.

Update 2: According to https://docs.microsoft.com/en-us/exchange/mail-flow/test-smtp-with-telnet?view=exchserver-2019 I followed all steps and it worked well (no refusals, mail queued for delivery).
If I do the same with the second server I get:

450 4.1.0 Don't use the Backup MX 'relay.rzone.de' while the Primary MX is available - please send your mail to ....

So the second server refuses because it supposes that the first is available. But why doesn't it work "normal way" (while it works with telnet)? The domain user still can't send mail to one address he needs to communicate with.
We send thousands mails and just one domain has this problem.

Improve this question
4
  • 1
    How did you determine, or how do you know, one server is good and the other is bad? Also, have you tried to manually establish an SMTP session to both servers?
    – joeqwerty
    Commented Jun 10, 2021 at 21:41
  • I use for the test centralops.net/co I see there both MX records : 10 somedomain.mail.protection.outlook.com 100 relay.rzone.de And the test mail from there passes all checks and is going through the first MX In the Toolbox of Exchange I can find a kind of trace and I see there that Exchange try to send hthe mail to the second server and after many attempts get status FAILED
    – Alexander P
    Commented Jun 11, 2021 at 6:54
  • Hi, any update?
    – Ivan_Wang
    Commented Jul 7, 2021 at 9:41
  • I am working through the same scenario now. It seems that at the time a specific remote Microsoft server was attempting a connection to my primary MX '10', this MX was down. So their side used a DNS lookup for the secondary MX '20' ... and they locked that MX host/IP for subsequent mailing. Later, something went wrong with the secondary '20' = I'm working on that now. I found out about this defect because that Microsoft server still has the secondary system '20' cached. So their mail is getting bounced but everyone else's comes in on the primary. HTH
    – TonyG
    Commented Jul 26, 2022 at 0:28

1 Answer 1

Reset to default
0

Normally, if domain will have several MX records, one of which is intended as a "backup" - with a higher preference number so that it would not normally be picked as the target for email delivery. In the case of errors from the lower-numbered hosts, sending email servers will deliver to the "backup" host: Source

For Exchange server, as far as I know, there seems to be no settings to specify MX record of recipients for outbound emails.

In your scenario, the lower-numbered host "MX 10 good.server" should have a higher priority to receive emails, I suspect there is something happening on it. As joeqwety said, you might need to manually establish the SMTP session between your Exchange server and destination servers.

Besides, the NDR(Undeliverable message) in sender's Inbox folder should tell us some reasons to cause a delivery failure(e.g. Enhanced Status code).

Improve this answer

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .