To change permissions on mailboxes navigate to the following path:
Click on the Service Management link and then select Mail Boxes tab. Click on the mailbox name for which you wish manage the service features.
Select Permissions tab. Here you will see the three permissions of sending emails:
Send on Behalf
Send As
Full Access
Select mailbox from the drop down list and then click on Grant Send On Behalf, Grant Send As and Grant Full Access button.
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