Hi, As others have mentioned, this is a long-standing issue with Outlook.com that seems to be related to internal DNS timeouts.
We’ve been able to resolve it by opening a ticket at https://olcsupport.office.com/, providing the rejection details, and repeatedly requesting escalation. Once it was escalated, it took several days of back-and-forth to demonstrate that the DKIM signature was valid. We even provided full SMTP logs showing that the exact same message, sent again less than one minute later, was accepted normally. Eventually, we received a response like this: Hello, > Thank you for contacting the Outlook.com Deliverability Support Team. > We have whitelisted the domain (****.COM). Please try sending emails and > let us know if you’re still seeing the NDR. If yes, provide us with the > latest NDR along with the domain. I would much rather see Microsoft fix the underlying issue instead of relying on manual whitelisting, but that seems to be their solution at this moment. Since we manage multiple domains and new ones are constantly being added, our current mitigation is: - Configure our MTA to ignore that 550 error and retry - Maintain an automated process to open support tickets requesting domain whitelisting Hope this helps. Best Regards, Fernando M M On Fri, May 8, 2026 at 4:13 PM John Levine via mailop <[email protected]> wrote: > Here's a summary of an SMTP rejection from Microsoft. I am quite sure > that the message had a valid arxiv.org DKIM signature and was sent > from an address in the SPF record. Anything I can do about it? > > We've been sending this person mail for years at an outlook.com.au > address and nearly all of them have worked, including several today. > > 550,5.7.515,Access denied, sending domain ARXIV.ORG doesn't meet the > required authentication level. The sender's domain in the 5322.From address > doesn't meet the authentication requirements defined for the sender. To > learn how to fix this see: > https://go.microsoft.com/fwlink/p/?linkid=2319303 Spf= Pass , Dkim= Fail > , DMARC= Pass [SY8P300MB0731.AUSP300.PROD.OUTLOOK.COM > 2026-05-08T18:21:38.150Z 08DEAD0640C37A36] [ > MW4PR04CA0059.namprd04.prod.outlook.com 2026-05-08T18:21:38.232Z > 08DEACE91EBF534A] [MW1PEPF0001615B.namprd21.prod.outlook.com > 2026-05-08T18:21:38.253Z 08DEACF5EA686851] > > R's, > John > _______________________________________________ > mailop mailing list > [email protected] > https://list.mailop.org/listinfo/mailop >
_______________________________________________ mailop mailing list [email protected] https://list.mailop.org/listinfo/mailop
