LDAP directory in Outlook mail 2002

I’m currently having difficulties in getting LDAP directory to work in MS Outlook 2002. It works well with other email clients such as Pegasus Mail, Outlook Express and Webmail except for MS Outlook. When using the Directory to find a contact, I get the following error messages:-
“Unavailable critical extension”
and
“The could not be completed. MAPI_E_CALL_FAILED.” The problem is not machine specific it occurs on machines that are using Outlook mail. Any thoughts on why it is happening and how it can be resolved. Thanks.