We've observed that this frustratingly generic error message sometimes
occurs when Microsoft modifies malformed header fields (such as missing
or empty fields, or 8-bit characters that aren’t properly MIME-encoded).
In these cases, it appears Microsoft corrects the headers before
performing DKIM verification, which then fails because the signature no
longer matches the altered content..
That said, this isn’t the only cause of the error. Even senders with
properly implemented DKIM and compliant headers can see a small
percentage of these bounces.
It's also possible that internal DNS resolution issues on Microsoft’s
side contribute to the problem.
Le 20/06/2025 à 23:00, Mark Alley via mailop a écrit :
It's the exact same problem as the other mailop thread "Outlook.com:
intermittent DKIM failures".
It's not you, it's them.
- Mark Alley
On 6/20/2025 3:36 PM, Mark Fletcher via mailop wrote:
Hi All,
We're seeing instances of emails being rejected by Microsoft with
DKIM errors, and I have no idea why. It's happening to maybe 1% of
the email we send to Microsoft properties (outlook.com
<http://outlook.com>, hotmail.com <http://hotmail.com>, live.com
<http://live.com>, etc). The error we get is:
550 5.7.515 Access denied, sending domain GROUPS.IO
<http://GROUPS.IO> 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
These are email group messages. We re-write the From line for all of
them, so they all have the @groups.io domain. For a given list
message, if it fails for one Microsoft recipient, it fails for all of
them.
I dump out these emails and run two different DKIM signature check
programs on them, and they pass.
Does anyone have any suggestions for things I should check?
Thanks,
Mark
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--
Olivier
Postmastery
*Email Infrastructure, Analytics and Deliverability*
Amsterdam, NL / Paris, FR
www.postmastery.com <https://www.postmastery.com>
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