Spam to postmaster account

After closing off the usual routes of ingress, I am left with a small
but still annoying number of spam emails addressed to "postmaster".
Today's batch appear to all have the same style and several are close
repeats of each other. Postfix accepts these regardless of my other
checks in order to be compliant with RFC 2821 (4.5.1).

Is any one else seeing this trend, and does anyone have any suggestions
for a RFC-compliant way to control it ?

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