Random thoughts and helpful IT tidbits
It can be annoying when you want full rights to a mailbox but it not to show up in outlook for reasons (cached exchange mode, only need to see one folder, etc). Adding full access through the ECP automatically adds this. To remove it, you need to use PowerShell.
You can do the first command with samaccountname
Add-MailboxPermission -Identity MailboxToOpen -User UserWhoNeedsToOpen -AccessRight FullAccess -InheritanceType All -Automapping $false
Don’t forget the Send As permissions as well. This command requires the Identity to be the Display Name of the AD User:
Add-ADPermission -Identity "Mailbox To Open" -User UserWhoNeedsToOpen -AccessRights ExtendedRight -ExtendedRights "Send As"