Cancellation of sending mail via SMTP, how to implement?

Question

SMTP

The script (someone else's) sends via SMTP and you need to make sure that the mail that is sent to the specified server does not leave it, it is better that it be deleted immediately.
In other words: the script sends an email via the specified SMTP on behalf of [email protected] But it is necessary that the client receive mail on behalf of [email protected] There will be several such servers, and you need to have rules somewhere that select the desired SMTP depending on the label in the subject line.

Answer 1

You need to intercept and delete the message BEFORE it goes to the addressee. Or delay, and send from another SMTP, depending on the recipient - generally ideal.