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Met this situation at work where we use Office 365 with a third-party email gateway and a custom domain name.

We received a complaint that an expected email is not received. This particular email has a PDF attachment. Checking the gateway log, we found that the email is delivered to the two CC addresses, but the one for the To address is absent.

After making sure the address is correct, the sender tried again without CC, and reported that the email is sent successfully, but we again didn't see the retry email even on gateway log. Then the sender used a Gmail account to send the email and we received it.

The CC's are properly received so I assume: 1) the server configurations are correct; 2) the domain names are fine; 3) the network is fine. The failure on retry disproved my guess that the email is lost by random failure on transit.

What failure could have caused this? How may I fix this?

Edit: The To and the CC's are of the same domain.

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  • Do you have any Mail Flow rules on your mail server? Are those recipients you added in To and CC in the same domain or in different domains? In order to further confirm your issue, it is suggested for you to try sending emails without PDF to recipients with or without CC and see if the issue has any differences.
    – Jeff Yang7
    May 26, 2021 at 2:58
  • Thank you for the comment! 1) No there are no Mail Flow rules; 2) To and CC were in the same domain, which is what made it confusing. The situation became clear when more hosts failed to receive emails and was resolved by correcting a DNS configuration error. I guess the sender flushed DNS cache right in between sending the CC and sending the To?
    – Yufan Lou
    May 30, 2021 at 23:52

1 Answer 1

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Self answering as we have fixed the problem.

The problem turned out to be a DNS configuration error, but it took an hour or two for other hosts to also have problems receiving emails.

The takeaway is that, do not assume the DNS is fine based on successful receipt of email, since the DNS cache may delay the onset of the problem. Check the configuration.

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  • Grateful to your sharing. Warm regards.
    – Jeff Yang7
    May 31, 2021 at 7:41

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