It (PasswordExpirationDate) is not, in fact, supported by the LDAP provider as referenced by: Provider Support of ADSI Interfaces.

It is supported by the WinNT provider, as well as MaxPasswordAge and PasswordAge. What if you used LDAP to grab the container you wanted to query to grab your 'user list' and then used the WinNT provider to determine password age using a method similar to my last post in this thread... get @pwage from user list and send notification e-mail?