> On 15 Oct 2026, at 12:31, 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 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…
I think it’s relatively new, we only started seeing this recently. I think Microsoft/Outlook changed something in the way or order they process their incoming mail. We allow users to add forwarding addresses on their mail, letting postfix handle the actual forwarding through our dedicated forwarding servers. We use SRS, but the body/DKIM stays intact. So normally DMARC still validates. And for most mail that still works fine. Only when the sender has duplicate headers AND signs that header with DKIM Microsoft now refuses them. Cor _______________________________________________ mailop mailing list -- [email protected] To unsubscribe send an email to [email protected]
