On 13.10.26 13:04, Cor Bosman via mailop wrote:
Hi all, we’ve noticed that outlook is returning 4xx for forwards with a
valid DKIM because there is a duplicate signed header. Usually a
Mime-Version header.
Microsoft now verifies the mime structure before DKIM so these fail
DMARC despite a valid DKIM. We’re seeing this with pretty large senders
like TikTok, Volkswagen, and many others.
How do other people deal with this? “Sender problem”?
On 14 Oct 2026, at 19:06, Matus UHLAR - fantomas <[email protected]> wrote:
Yes, I'd say it's a sender problem.
Do those senders always send messages with multiple Content-Type: or
Mime-Version: headers?
I guess it does not affect you when they send mail to Microsoft directly, only
when you forward their mail to Microsoft, correct?
On 15.10.26 10:53, Cor Bosman via mailop wrote:
It’s mostly Mime-Version. Maybe it’s some mail management software that
creates these duplicate headers. It’s only on forwarding yes.
I would think this doesn’t only affect us, but everyone that does
forwarding in any kind of volume.
now the question is who does the forwarding - I have never heard of this
problem.
if it's .forward in users' directories or similar feature of MTA etc...
when guessing, I can speculate about Mime-Version not being properly
formatted, missing cr/lf etc...
--
Matus UHLAR - fantomas, [email protected] ; http://www.fantomas.sk/
Warning: I wish NOT to receive e-mail advertising to this address.
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