I'm working on an email marketing tool and so far we've been recommending our customers to set up an SPF-record (Sender-ID) and a DKIM-record, we also have our own SPF-record on the mail server and a shared DKIM-record for those who do not set up their own DKIM-record.

Those that do not set up their own DKIM-records still pass the DKIM-test, but with the notice that "identity doesn't match any headers" (according to port25), i.e, it doesn't match the textual sender domain.

But does anyone know if this "discrepancy" actually has any impact on spam scoring/probability, i.e, should we continue to recommend our customers to set up a DKIM-record (as opposed to just using our shared) or is just wasted effort?