[ietf-dkim] The mystery of third party signatures

[John Levine]

>In light of the comments by Bill Oxley and my belief that the ability of
>a domain to designate signing by a specified 3rd party is useful, ...

It would really be helpful if you two could explain WHY you think it's
useful.  Given the way that DKIM works, there's only two possible
benefits from third party signatures.  Say we want to have isp.com
signing for its customer a.com:

A) a.com sends its mail through isp.com's system, a.com is unable to
sign mail before it's relayed to the smarthost, and it's too hard for
isp.com to apply an a.com signature

B) Nobody's heard of a.com, so it wants to benefit from the reputation
of isp.com.

If you can't tell us which of these you have in mind, we're just going
to go around in circles.  I don't think there's any other problem that
DKIM signatures can solve, but if you disagree, please tell us what it
is.

R's,
John