On Tue, Oct 13, 2026 at 01:21:36PM +0200, Benny Pedersen via mailop wrote:
> Cor Bosman via mailop skrev den 2026-10-13 13:04:
> > 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.
Please provide an example, headers and esp signature.
Mime-Version is not defined with "at most one" as far as I can see. RFC
2045 does not talk about it and the IETF registry does also not record
such a restriction. Only headers defined directly in RFC 5322 show
restrictions.
> eq mailop.org
> multiple To: signed
Signing non-exsting headers is called oversigning. It protects from
adding new headers.
Bastian
--
Landru! Guide us!
-- A Beta 3-oid, "The Return of the Archons", stardate 3157.4
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