Outlook users cannot send to my mail server

This one is unusual: A customer of Outlook mail (365 I guess) is having issues sending to me (my mail server is on premise).

Some messages are getting delivered after 12 or even 18 hours. Others are returning as NDRs:

3/31/2023 6:54:00 AM - Server at AM0P194MB0499.EURP194.PROD.OUTLOOK.COM returned '550 5.4.312 Message expired, DNS query failed(ErrorRetry)'
3/31/2023 6:43:59 AM - Server at mydomain.com ( returned '450 4.4.312 DNS query failed [Message=ErrorRetry] [LastAttemptedServerName=mydomain.com] [VI1EUR05FT043.eop-eur05.prod.protection.outlook.com 2023-03-31T06:44:00.009Z 08D<snip>62](ErrorRetry)'

When the mail is delivered late, I can see from the headers that it is spending some hours inside the Outlook network (but given how unhelpful Microsoft’s support is, I am posting here).

I’m receiving fine from other services/domains.

The issue started about a month ago, and the error is the same: DNS query failed

I’ve been using Cloudflare for years, and the set-up is stable.

I’ve reviewed the records, and, testing mydomain.com with mxtoolbox.com shows no problems. SOA is OK also.

However, I’m not sure if this is relevant: There are a couple of glue records configured on the registrar (not Cloudflare) (e.g. ns1.mydomain.com) that point to IPs, but there aren’t any A, AAAA, nor CNAME records for these hostnames. Is this a problem?

(IP is obfuscated intentionally)

MX records cannot point to a CNAME, the target MUST be to A/AAAA resource records.

I believe that this is sometimes ignored by MTAs, so it might work sometimes, and not others. Pointing the MX at mercury will bring your setup in line with the specification.



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