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1f0cfd4662
Using postfix to handle the incoming email gateway complicates things a great deal: - It cannot verify that incoming email addresses exist in Zulip before accepting them; it thus accepts mail at the `RCPT TO` stage which it cannot handle, and thus must reject after the `DATA`. - It is built to handle both incoming and outgoing email, which results in subtle errors (1c17583ad5,79931051bd,a53092687e, #18600). - Rate-limiting happens much too late to avoid denial of service (#12501). - Mis-configurations of the HTTP endpoint can break incoming mail (#18105). Provide a replacement SMTP server which accepts incoming email on port 25, verifies that Zulip can accept the address, and that no rate-limits are being broken, and then adds it directly to the relevant queue. Removes an incorrect comment which implied that missed-message addresses were only usable once. We leave rate-limiting to only channel email addresses, since missed-message addresses are unlikely to be placed into automated systems, as channel email addresses are. Also simplifies #7814 somewhat.