The "From:" header should not be signed if it contains more than one
sending address. ...
Does anyone see such a statement as causing a problem?
I see it as needless and futile micromanagement. The point of a DKIM
signature is that the signer is taking responsibility for the message.
The only semantics that a DKIM signature has is "blame us if you don't
like this message." That's it.
We don't know all of the reasons that a signer might legitimately want
to sign multiple From: addresses, nor do we know all of the ways that
a bad guy might try to trick someone into signing his message, with
multiple From: addresses being rather low on that list.
I could easily imagine an SSP-like system limiting itself to a subset
of otherwise syntactically valid messages, e.g. only one address in
the From: line, sender matches signer, or any of a host of other
rules. But for the basic DKIM, a signer can sign anything he's
willing to, and please leave it at that.
R's,
John
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