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Or you just use SMTP and read the 200 response on the SEND?

In extension to that spirit, some SPAM could be eliminated, if more people would turn address verification on in their SMTP servers, which makes the delivery peers symmetric.

Do you mean source or destination address verification or both?

Source address verification doesn't really mean anything (no-reply@example.co.uk) and destination verification is obvious and as far as I am aware pretty much no-one doesn't do it already.

"delivery peers symmetric" - what does that mean?

With source address verification (and server validation) it is guaranteed that the mail comes from the server that controls the senders mail address and that this address does indeed exist. With symmetric I mean that both servers then resolve each other the same, both check whether their side of mailbox exists and they share the time during which this happens, so you can't use it for DOS, since it takes your time as well.

You send me mail with noreply@example. I go to your MX to see if noreply@example will receive mails. If not you are spamming.

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