Lightweight Directory Access Protocol (LDAP), is an internet protocol used to look up contact information (email addresses) from LDAP email directories. Many email client applications can be configured to look up contact information, from such directories.
To facilitate this, your email client application will require the address of an LDAP server. When you attempt to look up contact information, your email client will attempt to get this information from the server specified.
The LDAP server setting is not a mandatory setting and will not affect the transmission or receipt of your email. At the same time, you will not be able to use LDAP services without a server specified in the configuration of your email client application.