Is there a way to tell Spamassassin to skip a mail, in case a certain header is present, so it leads to an unmodified output of the original mail? So that no Spamassassin-Headers are added, no Bayesian learning is done, etc. Just completely ignore it and return it like that when sent through Spamassassin.
I don't know if there is a way to do that in Spamassassin. You can, however, first pass the mail to a separate filter which can check for that header and then either pass the mail to Spamassasin or do a clean passthrough if the header is not present.