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 13.10.26 15:04, Cor Bosman via mailop wrote:
I also just tested it with Content-Type, same problem. So likely also
other headers.
But these headers have to actually be duplicated. Im guessing mailop
itself doesn’t do that, it only over signs. Many senders do add duplicate
headers, and we mostly see this with Mime-Version for some reason.
My guess, MS first “fixes” the header to remove the duplicate, and then
verifies the signature, which then obviously doesnt validate anymore. I’m
inclined to say “sender problem”, but this is happening with some pretty
large senders.
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?
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