> 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)"
Domainkeys is long since deprecated. Also, the DKIM signature is
reported as OK, so that's not really a good example.
In any case, many mailing lists break DKIM sigs by modifying the subject
line or body of messages, so rejecting/discarding mail based on DKIM
alone is prone to cause trouble for you. DMARC offers an approach that
also includes SPF, but it has problems of its own.
> is that something that can be rejected/blocked in Postfix, and how? or
> where should that be utilized ?
You appear to be using amavis, so I suggest you use amavis' spam scoring
mechanisms instead of Postfix.