> On 14 Oct 2026, at 19:06, Matus UHLAR - fantomas <[email protected]> wrote: > > 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?
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. Cor _______________________________________________ mailop mailing list -- [email protected] To unsubscribe send an email to [email protected]
