domain (ip ip6) {
    table filter {
        chain INPUT {
            # SMTP for server communication.
            proto tcp dport 25 ACCEPT;
            # SMTP for client submission (with alternative port)
            proto tcp dport 587 ACCEPT;
            proto tcp dport 26 ACCEPT;
            # IMAP
            proto tcp dport 143 ACCEPT;
            proto tcp dport 993 ACCEPT;
            # ManageSieve
            proto tcp dport 4190 ACCEPT;
        }
    }

    table nat {
        chain PREROUTING {
            # Set-up redirection for alternate SMTP submission port (to avoid
            # ISP/hotel blocks etc).
            proto tcp dport 26 REDIRECT to-ports 587;

            # Set-up redirection for alternate SMTP port (to avoid ISP blocks).
            proto tcp dport 27 REDIRECT to-ports 25;
        }
    }
}