The Messenger Express Client supports the concept of a Personal Address Book (PAB). This enables users to store personal contacts (for example, business contacts, friends, and family) in the Directory Server. Each time a new personal contact is added to the user’s PAB, a write is made on the Directory Server. If you do not take these actions into consideration, the LDAP Master Server can face heavy loads (regardless of the Directory replication strategy).
One method to avoid performance issues on the User and Group Directory Server is to place the PAB information on a separate Directory Server. This enables PAB interactions to avoid placing a load on the LDAP Master Server.
If you are running both the current Communications Express client and also the deprecated Messenger Express Web mail interface, the address books used by these two clients do not share information. If end users switch between the two client interfaces, the two address books will contain different entries.