Given our company structure I frequently have to allow User A to read or send as User B. This task is simple enough with the Exchange Management Shell (EMS) but does require two separate commands.
The first command here allows full access to the mailbox:
Add-MailboxPermission "FirstName LastName" -User "UserName" -AccessRights FullAccess
The FrirstName LastName is the mailbox that you are adding permissions to. The Username is the user that will have the permissions that you are adding.
The second command you will need to run is only necessary if you need to send email as that user.
Add-ADPermission -Identity "FirstName LastName" -User "UserName" -ExtendedRights Send-As
Once again the FirstName LastName should be the mailbox that you are adding permissions to and the UserName is the user that will have the permissions you are adding.
Based on how often I have to do this I really need to write a quick little application that will allow me to add these permissions with ease instead of having to type out the commands every time. I will keep you updated if I do.
*You have to have Exchange Organization Administrator role privileges on the Exchange server to make these changes.