[ietf-dkim] Re: 3rd party signing

[John L]

>> It may well be true that you only sign third party mail, but I still
>> don't understand what use a recipient might make of that information.

>> If they get unwanted mail from someone and you've signed it, they'll
>> complain to you regardless.

> A recipient will then have a valid party to complain to which is better
> than blocking a domain that has been spoofed.

I still don't understand the scenario.  Let's call the domain isp.com.
Is it:

A) No mail has an isp.com From: address, but mail with other From: 
addresses may have an isp.com signature.

B) Mail goes out with From: addresses at isp.com, but none of it is 
signed.  Mail with other From: addresses may have an isp.com signature.

C) something else.

Scenario A is "we send no mail," with the possibility of third party
signatures on other mail being irrelevant.  Like I said, if you sign it, 
you'll get the complaints no matter what your SSP says.

Scenario B is technically possible but makes no sense.  If you have the
ability to sign mail, why wouldn't you sign your own?

What am I missing here?

R's,
John