Per client/helo/sender/recipient UCE restrictions
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The Postfix SMTP server allows you to specify UCE restrictions on
the right-hand side of SMTPD access tables, so that you can have
different UCE restrictions for different clients or users.

The only anomalies in this scheme are that (1) message header_checks
and body_checks are still the same for every message, and (2) you
must use a restriction class name (see below) if you want to specify
a lookup table on the right-hand side of an access table (this is
because Postfix needs to open those tables ahead of time).

Restriction classes allow you to give easy-to-remember names to
groups of UCE restrictions (such as permissive, restrictive, and
so on).  For example in main.cf:

    smtpd_restriction_classes = restrictive, permissive
    restrictive = reject_unknown_sender reject_unknown_client ...
    permissive = permit

With this in place, you can use "restrictive" or "permissive" on
the right-hand side of your per-client/helo/sender/recipient SMTPD
access tables.