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 (0.0.0.0) 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.

https://www.rfc-editor.org/rfc/rfc2181.html#section-10.3

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