sendmail.net: RFC 1777: Lightweight Directory Access Protocol (LDAP)

“What does sendmail 8.10 have in common with Microsoft
Outlook employee directories and the Yahoo interface on your cell
phone? They all use – or at least can use – LDAP to access the huge
directories of information they need.”

“LDAP is the IETF’s pragmatic reaction to the much more
comprehensive X.500 directory standard established in 1988 by the
International Telecommunications Union. X.500 already specifies a
Directory Access Protocol, but real-world engineers in the early
1990s wanted a way to communicate with X.500 directories without
consuming bandwidth… and without having to support complex data
structures. LDAP works by sending requests directly on top of TCP,
rather than adding extra layers for session state and data
presentation into each packet. LDAP also uses plain text strings in
place of complex data objects. Not only do these low-tech
approaches save bandwidth, they make LDAP simpler for less
technical administrators and users to learn and work with.”

“LDAP has become so successful and ubiquitous as a means of
accessing information that sendmail 8.10 has incorporated LDAP
support to let administrators replace standalone configuration
files with a central directory service.”