LDAP external directory lookup MTA options

There are options controlling the LDAP fundamentals of any   lookups the MTA any be configured to perform. Such lookups, hence such options, were added for Messaging Server 7.2-7.02.

While the feature, new in Messaging Server 7.4-18.01, to allow explicit host and port specifications in LDAP URLs configured for MTA use means that these options might not all seem quite as necessary -- in operation since an external LDAP directory will likely require different bind credentials than an internal LDAP directory, as well as potentially other different settings, it is generally still useful to specifically configure using   lookups instead of ldap:.

Example uses for LDAP external directory lookups -- LDAP lookups directed against some LDAP directory other than the one used for regular user/group data -- would be cases where Sieve external lists are  stored in some  external LDAP directory, or for cases of looking up some data in  mapping tables from an LDAP directory other than the usual LDAP directory.

See also:
 * ldap_ext_host MTA Option
 * ldap_ext_max_connections MTA Option
 * ldap_ext_password MTA Option
 * ldap_ext_port MTA Option
 * ldap_ext_username MTA Option
 * Sieve external lists
 * MTA URL types
 * MTA options
 * Mapping entry templates