Header From: addresses and other backwards-pointing addresses and forwards-pointing header addresses receive one additional processing step. This additional processing step is referred to as address reversal, and can be performed via LDAP lookups, and/or via use of a reverse database and/or REVERSE mapping. Section 3.3.1 discusses use of LDAP lookups for address reversal; Section 3.3.2 discusses use of the reverse database and REVERSE mapping. An LDAP lookup, if any, is performed prior to checking the reverse database and/or REVERSE mapping.