Attributes for Exchange 5.5 Mailboxes : Class
This is NOT an LDAP attribute of the mailbox class. It is rather an interface property of the ADSI object representing the mailbox. Therefore, this 'attribute' is not created by the LDAP directory but provided by the ADSI interface. For this reason you cannot use the 'Class' property in any LDAP filter!
The string contained here in this property represents the structural object class which is organizationalPerson for Exchange 5.5 mailbox objects.
|Data type||ADSI Interface Property -> String|
|Interface documentation||Microsoft - MSDN|
By evaluating the 'Class' property, you can check if an connected object is in fact a mailbox.
See also the explanations for the attribute objectClass. There you will find the base classes from which organizationalPersonis derived.