define contact mailbox

[JNTA]

When I try:
1 Dim user As New
DirectoryEntry(LDAP://server.dom.com/cn=contactTest,ou=Mailing,dc=dom,dc=com,
"user", "pass", AuthenticationTypes.Secure)
2 Dim mbx As CDOEXM.IMailboxStore = user.NativeObject
3 mbx.CreateMailbox(LDAP://CN=contactTest,ou=Mailing,dc=doc,dc=com)
4 user.CommitChanges()

I receive "System.InvalidCastException: Specified cast is not valid." line
2.

why???

I would want define the email address for the contacts.

thanks
dJa