MTAs, if configured correctly, do not talk to anything outside your network that is not specified by MX DNS records. Unless we are talking about emails traversi

MTAs, if configured correctly, do not talk to anything outside your network that is not specified by MX DNS records. Unless we are talking about emails traversing internal networks and even then competent admins do not transmit data in cleartext, so sniffing is out of question. Of course, some metadata is always leaked, but that is the reality of most internet protocols, HTTPs notwithstanding. Now if we're talking about backdoors, it's a game over in any scenario.