If this continues to occur after a

If this continues to occur after a reboot of your PC, this means you have not been authorised to access that Delegated Mailbox.
 
A request for permission to access the mailbox must be received via email only to the Service Desk, with your request supported by the mailbox’s ‘recognised authority’ (normally your line manager or your business area member of staff responsible for the original setup request and continued usage of the Delegated Mailbox).
 
If you believe you should have access already and you encounter difficulties with this process, please contact the Service Desk who will advise you further