Most organizations use a directory service to find email addresses of users who are not listed in local address books or not listed in corporate directories such as the Global Address List. One of the most often used directory service providers out there is known as a Lightweight Directory Access Protocol server, more commonly described as an LDAP server.

LDAP originally provided access to ISO X.500 directory service. But it has grown to also support various directory services, namespace, access protocol and rules. It also describes their interactions based on the X.500 model.

Clients can access the LDAP server and make requests for information contained in the directory. Email clients would be interested in locating email addresses of people who are not in the local directories. An LDAP server can also be used to answer requests from web browsers, management applications and other such clients. If the LDAP server queried cannot provide the information requested then it can forward the request to another LDAP server.

In the Microsoft world both Exchange Server and Outlook support LDAP. Email clients can submit queries in search of email addresses of users outside their internal directories.

There is an Office Customization Tool (OCT) which will allow an administrator to configure LDAP search actions such that certain searches are disabled or passed through custom filters. A Group Policy setting can also be used to limit LDAP searches. Custom filters can be implemented and provided to end users by changes made to either the Outlook Profile (PRF) file or by making changes in the registry.

One of the error messages that can be produced when working with Outlook and LDAP servers might happen when a user tries to open the Address Book and receives this message: “Can’t contact LDAP Directory server (81)”.

If Outlook has been configured to use Internet directory services (LDAP) and Outlook cannot connect to the LDAP server when Outlook attempts to access that particular address book that the LDAP server is responsible for then the above error will be generated.
 
This error message can be produced with Microsoft Office Outlook 2002, Microsoft Outlook 2003, or Microsoft Office Outlook 2007.

If you are using Microsoft Outlook 2002 or Outlook 2003 and you are not using Lightweight Directory Access Protocol (LDAP) Internet directory services then you can fix this problem by removing the LDAP address book. Follow the steps listed below:

1. From the Tools menu, click Email Accounts.
2. Under Directory, click View or change existing directories or address books, and then click Next.
3. Under Directories and Address Books box, click the LDAP address book that you want to remove, and then click Remove.
4. Click Finish.
5. Quit, and then restart Outlook.

If you are using Microsoft Outlook 2002 or Outlook 2003 and you are also using Lightweight Directory Access Protocol (LDAP) Internet directory services then you should verify that the LDAP settings are correct. Follow the steps listed below:

1. From the Tools menu, click Email Accounts.
2. Under Directory, click View or change existing directories or address books, and then click Next.
3. Under Directories and Address Books box, click the LDAP address book that you want to configure, and then click Change.
4. Verify that the settings for the LDAP server are correct. Make changes as necessary.
5. Click Next, and then click Finish.

If you are using Microsoft Outlook 2007 and you are not using Lightweight Directory Access Protocol (LDAP) Internet directory services then you can fix this problem by removing the LDAP address book. Follow the steps listed below:

1. From the Tools menu, click Account Settings.
2. On the Address Books tab, click the LDAP address book that you want to remove, and then click Remove.
3. Click Close.
4. Exit and restart Outlook.

If you are using Microsoft Outlook 2007 and you are also using Lightweight Directory Access Protocol (LDAP) Internet directory services then you should verify that the LDAP settings are correct. Follow the steps listed below:

1. From the Tools menu, click Account Settings.
2. On the Address Books tab, click the LDAP address book that you want to configure, and then click Change.
3. Verify that the settings for the LDAP server are correct. Make changes as necessary.
4. Click Next, and then click Close. 

For more information about Lightweight Directory Access Protocol go to the Internet Engineering Task Force (IETF) web site at www.ietf.org