ot: dkim "fail (message has been altered)" ?

[lists]

I'm attempting to implement dkim/dmarc, noticed that many spam messages
have like "fail (message has been altered)":

Authentication-Results: geko.sbt.net.au (amavisd-new);
        dkim=pass (1024-bit key) header.d=dossierinfotech.in.net;
        domainkeys=fail (1024-bit key)
        reason="fail (message has been altered)"
        header.from=mai...@dossierinfotech.in.net
        header.d=dossierinfotech.in.net

is that something that can be rejected/blocked in Postfix, and how? or
where should that be utilized ?

thanks,

Voytek