With DKIM, The Signer Domain says "I signed this mail". It does
not approve content, or state that content is benign. The receiver
decides whether to give this signature preferred treatment. There
is little or no controversy about this aspect of DKIM.
With SSP, The Sender Domain says "I send such and such mail": if
any is signed, or not signed. This is primarily relevant for mail
without valid signature by The Sender Domain. There is little or
no controversy about this aspect of SSP.
There is controversy about attempts by The Sender Domain to make
statements about mail handling practice by other parties such as
forwarders or receivers.
I see these as well-meaning attempts to provide a magic wand against
email forgery, just like the one that SPF attempted to provide;
attempts that are rooted in the assumption that The Sender Domain
can somehow constrain forwarder or receiver behavior.
In my opinion, credibility is determined by the combination of
Sender Domain and Signer Domain; different combinations having
different credibility, many combinations having none at all. If
SSP attempts to infringe on or otherwise constrain forwarder or
receiver practice, then it may very well become as relevant as SPF.
Wietse
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