[ietf-dkim] Delegating responsibility: a make vs. buy design decision

[Damon]

> In (1) the public key is listed under the author's domain, while
> the secret key is operated either by 1a) the author's MTA or by
> 1b) the signing party's MTA.  This is what I called a first-party
> scenario above. The verifier can't distinguish between 1a) and 1b)
> except by parsing the contents of Received: message headers.
>
> In (2) the public key is listed under the signer's domain, and the
> secret key is operated by the signing party's MTA. There is no
> relationship between author-domain and signing-domain. This is what
> I called a third-party signature above.
>
> In both (1) and (2) an assessment can be made on the basis of the
> the signing-domain. If I get mail with a signature from some no-name
> signing domain, then the author-domain (rfc822.from) is mostly
> irrelevant. And if I actually do have reasons to trust the
> signing-domain, then the author-domain is mostly relevant in case
> (1), and mostly irrelevant in case (2).

Wietse,
Thank you for the clearest definition I have seen to this point.
What is the mechanism for depreciating (2)?

Regards,
Damon Sauer