Unable to grant Full Mailbox Database Access to Service Account

Hi,

I have installed Exchange 2010 SP2 and am trying to grant full access to an ad user account (without a mailbox) so a service account can update calendars using a third party utility. I have tried a number of powershell scripts to do this but none of them are granted Full Access which I need. I'm sure these worked before so I'm unsure what has changed. Scripts I've tried so far are: -

Get-mailboxdatabase "Mailbox Database " | add-ADpermission -user serviceuser -accessrights genericall

Get-mailboxdatabase Mailbox Database | add-adpermission -user serviceuser -accessrights fullaccess -inheritancetype all

Get-MailboxDatabase -server YourServername | add-adpermission -user ServiceUser -accessRights GenericRead, GenericWrite -extendedrights Send-As, Receive-As -inheritanceType all

When I then try to check the access rights, I still haven't got Full Access. Some suggestions were to restart the AD Topology which I have tried as well as the Exchange Store and System Attendant but these don't work either. I do have a large number of mailboxes but even after waiting over 8 hours, there's still no change

Any ideas?

Regards

Chris