Address reversal

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After address rewriting via the MTA's rewrite rules, header From: addresses and other backwards-pointing addresses and forwards-pointing header addresses normally receive one additional processing step.1 This additional processing step is referred to as address reversal. Another term used is address canonicalization, since address reversal is most commonly used to change possible alternate address forms into a single, canonical form. Address reversal can be performed via LDAP lookups, and/or via use of a reverse database and/or REVERSE mapping. Note that an LDAP lookup, if specified via the reverse_url MTA option, is performed prior to checking the reverse database and/or REVERSE mapping.

Special handling of subaddresses is available during address reversal. And special handling of what might be termed address "decorations", namely RFC 822 comment strings and personal names, is also available.

New in 8.0, address reversal can be made sensitive to exactly which header field (e.g., From: vs. Sender:, etc.) is being processed by setting a special bit in the use_reverse_database MTA option which will cause inclusion of the header field name in REVERSE mapping table probes.

The primary use of address reversal is to substitute a generic, standardized address for internal or host-specific addresses. Address reversal is a particularly powerful tool when used in conjunction with aliases.

1Address reversal processing can be restricted in various ways. Address reversal can be restricted to only backwards pointing addresses if bit 2/value 4 in the MTA option use_reverse_database is cleared. Application of address reversal processing to envelope From address can be disabled using the reverse_envelope MTA option. The noreverse channel option can disable address reversal from being performed during enqueues to particular destination channels. However, in modern MTA configurations using LDAP-based aliases, a great many functions are critically dependent upon the MTA performing its LDAP lookup address reversal; before considering any restrictions upon normal address reversal, consider carefully the discussion of Intended side effects of LDAP address reversal.

See also: