> >> In some systems, sieve scripts and other filtering is done *after* the
> >> MUA drops the message in the delivery mailbox.  If that drop removes
> >> the signature, that hampers the sieve/filtering process severely.  A
> >> sieve "redirect" becomes impossible, and the filtering would not be
> >> able to use the DKIM signature for other purposes either (though it
> >> might be able to rely on the auth-results header field for some
> >> things.

First, I made a typo: where I said "MUA" above, I meant to say "MDA".

> Isn't it the MDA which runs sieve filters?

Two answers:
1. Most often, yes, but, as I meant to say above, *when* in the
process it does it is variable.  All filtering would have to be done
*before* the removal of the dkim-signature header field.

2. There are MUAs (email clients) that support sieve and other
filtering, and those necessarily run it *after* the MDA would remove
the signature.

> Transparent forwarding is
> harder for a MUA, as it usually has to go through Submission protocol,
> which can change a number of message details.

Can, but generally does not, other than to add a "Received" line or
two.  There's a long history of MUAs with "re-send" functions, which
have successfully done such transparent redirects.  Removal of the
signature would break that function with respect to DKIM (and,
therefore, DMARC).

Barry

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