For postfix, don't defer if "abused legit".  (I.e., DBL return code in
the 127.0.1.100+ range.)  This used to work for Postfix 3.1.14 (Stretch)
but for 3.4.8 (Buster) the 'defer_if_reject' also applies to
$smtpd_relay_restrictions, to reject_unauth_destination &
reject_unlisted_recipient in particular.

On the infrastructure boundary.  We don't reject/quarantine as it would
affect members who forward their mail sent to <user@example.com> to
<user@fripost.org>.  Members can install Sieve rules to send any
messages with failed Authentication-Results headers directly in their
spambox.

Unlike what we wrote in 2014 (cf. 4fb4be4d279dd94cab33fc778cfa318b93d6926f)
the postscreen(8) server can run chrooted, meaning we can also chroot
the smtpd(8), tlsproxy(8), dnsblog(8) and cleanup(8) daemons.

These addresses need to be accepted on the MX:es, as recipients
sometimes phone back during the SMTP session to check whether the sender
exists.

Since a time-dependent suffix is added to the local part (cf.
http://www.postfix.org/postconf.5.html#address_verify_sender_ttl) it's
not enough to drop incoming mails to ‘double-bounce@fripost.org’, and
it's impractical to do the same for /^double-bounce.*@fripost\.org$/.

‘noreply@’ aliases can be added by routing them to
‘@discard.fripost.org’.

In particular, since Postfix is now able to perform LDAP lookups using
SASL, previous hacks with simble binds on cn=postfix,ou=services,… can
now be removed.

It has to be performed last, to give a chance to be accepted as a
regular mailbox.

We introduce a new, dedicated, smtpd daemon whose only purpose is to
resolve catch-alls.