On 2018-09-02 06:52, J. Fahrner wrote:
> 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.
In which case write a rule in which emails from A={gmail,yahoo} with a
reply-to to a different B={gmail,yahoo} address get blocked (your
decision whether to block only when A == B etc.) … no reason to prevent
legitimate reply-to use cases such as those mentioned by others above,
and such a rule against the big internet mail sites is likely to hit
only a very small proportion of those legitimate users! :)

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Bill Gallafent