Intended side effects of LDAP address reversal

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Doing a reverse_url lookup actually has effects beyond pure address reversal. (And this is why a reverse_url lookup normally uses the $R substitution for a filter that searches for a given address as the canonical mail attribute, as well as searching for the attributes that would actually require address reversal: one wants the reverse_url lookup to find an entry even for an address that is already in canonical form.) The recommended setting for the reverse_url MTA option makes use of the LDAP URL $N substitution to specify an extensive list of attributes to be fetched; so reverse_url lookups also normally make use of (or at least fetch and cache) the attributes named by the MTA options:

The recommended setting for the reverse_url MTA option also uses the $V substitution for locating the domain in which the sending user address is located. Because of this implied lookup of the sending user's domain, the MTA's message processing can then also make use of per-sending-domain LDAP attributes including those named by the MTA options:

Note: In actual operation, the MTA and domain map caching of domain lookup results means that the domain attributes are often available from a cache, without need for an additional actual LDAP query at this point. That is, while the reverse_url caused fetching of the sending user's personal LDAP attributes is relatively likely to involve a query all the way to the backend LDAP server, the "fetching" of the sending user domain LDAP attributes is often short-circuited, with the domain attributes cached due to a prior lookup.

So the list of potential side-effects resulting from address reversal, when it is properly configured to fetch these various per-sending-user and per-sending-domain LDAP attributes, is quite extensive, including effects on message size limits, message recipient limits, conversion tags, message capture, spam/virus filter processing opt-in, archiving opt-in, source channel "switching", and (if a notification message must be generated), notification language preference, non-return-of-content in notification messages, and per-domain postmaster address selection, etc.

New in the 8.0 release, bits of the use_reverse_database MTA option can be set to disable use of either the envelope From (MAIL FROM) address, or the authenticated sender address, for purposes of source-based message size or recipient limit settings, as well as capture actions.

See also: