Adding Additional Mailbox trouble

[BCasita]

.. . .
On Exchange 5.5 went to "Helpdesk mailbox" added userids.

On User Manager for Domains added userids to Helpdesk Groups.

Then, in the Helpdesk Outlook client "mailbox" added users as delegates
with
rights & role for folders.

When I go to the USERS Outlook - Tools, Services, Microsoft
Exchange Server, Properties, Advanced Tab, select Add
- get message that can not add user (?).
( Name can not be resolved ).

I have to get this resolved.

I have to be missing something in Exchange, right?