Quoting J. Fahrner (jf@???):

> Nowadays lots of spam is sent through freemailers using a disposable
> email address and a reply-to to a different freemail address. Since
> you cannot block the whole google and yahoo mail servers, the only
> way to reject such spam is by the reply-to header.

Jochen --

It is in my view an extremely unwise course of action to reject spam
after MTA acceptance, because you are very, very likely to then commit
backscatter spam, thereby worsening the problem.
https://en.wikipedia.org/wiki/Backscatter_(email)

The alternative approach I strongly recommend is, to the extent
possible, detect and reject spam during the pending SMTP delivery
attempt, i.e., not 220 Accept it in the first place but rather issue a
55X Die Spammer Die SMTP response.

If mail has already been accepted and is later determined to be spam,
then IMO it's too late to reject it.

My SMTP installations have been implementing that strategy since the
early 2000s, and I continue to strongly recommend it.

(People who don't operate their own MTAs have rather poor opportunity to
detect and block spam, but they too should not worsen the backscatter
problem by attempting to reject spam from MDA or MUA software.)