Is there are bug in the address book of the R6 Client? When you are creating a new mail and click the button “Address” and changing the view to “Notes name hierarchy” you can choose users.
But, when you click “OK” or “Cancel” and open the address dialog a second time (perhaps you forgotten one user) notes remember the sorting “Notes name hierarchy” and you cant choose more users in this view! The message is “The selected entry does not have a valid address”!
When you change the view into the standard view you can choose other people.