When DKIM validation works correctly at other MBPs but fails at Outlook (with 550 5.7.515 Access denied ... Spf= Pass , Dkim= Fail , DMARC= Pass) , it is typically due to one of two types of issues:
- Sender-side issues: These may include improperly encoded non-ASCII characters (such as é, ç, è, à) in header fields, missing or duplicate headers, or other non-compliance with RFC standards. In such cases, Microsoft first fix the message to correct header compliance before performing the DKIM check, which will then fail due to these alterations. When this happens, the bounce rate is usually quite high.
- Microsoft-side issues: There is also a known scenario where the issue originates from Microsoft’s infrastructure. In some cases, their systems fail to validate DKIM, likely due to internal DNS resolution problems (it is unclear whether using a 2048-bit key or CNAMEs exacerbates the issue).
These cases are typically rare and, based on experience, account for roughly an additional 0.1% of bounces.
Microsoft is reportedly aware of the problem and working on a fix.
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Olivier
Le 06/05/2026 à 08:57, Hello Maildrop
via mailop a écrit :
Hello,
One of my customer, hosted at Google workspace, have recurrent problem when sending to some addresses at live.fr or hotmail.com
The messages bounce back indicating a DKIM failure, however the messages sended are well signed. Delivery to other destinations enforcing DKIM/DMARC does not cause any problems (Gmail, Orange)
550 5.7.515 Access denied, sending domain MISSEGLE.COM 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 [AM8P189MB1234.EURP189.PROD.OUTLOOK.COM 2026-05-06T05:21:48.296Z 08DEAAD6427386FD] [DU2PR04CA0024.eurprd04.prod.outlook.com 2026-05-06T05:21:48.305Z 08DEAAEBE699717D] [DB1PEPF000509FE.eurprd03.prod.outlook.com 2026-05-06T05:21:48.308Z 08DEA716CB8BDD0A]
I can't identify the cause of this problem, do you have any ideas?TOUREL David
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