Linda, imagine that we have 2 spam filters only: SpamAssassin and DKIM.
As I understand, SM must process all emails with all spam filters. And indicate the results in the headers of all emails.
So, we have to see both SpamAssassin and DKIM in every message.
Another possible scenario is: SM runs all the filters (SpamAssassin and DKIM), but mentions in the headers only those of them which are not 0. In this case some email will have SpamAssassin (if its score is not 0) in their headers, others DKIM, others both of them or none of them.
But in the reality SM may write:
SpamAssassin 0 [raw: 0]
, but may also not mention SpamAssassin at all.
I do not understand why.
Does it process every message with SpamAssassin? If not - why? If yes, then why does not mention SpamAssassin score in every email?