You use the Send on behalf of send permissions to specify which users can send messages on behalf of the mailbox owner.
To customize send permissions for mailboxes
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In the Manager, select the Azure Active Directory > Mailboxes category. 
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Select a mailbox in the result list. 
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Select the Assign send authorizations task. 
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Select the table which contains the user from the menu at the top of the form. You have the following options: - 
Mail-enabled distribution groups 
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Mailboxes 
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Mail users 
 
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In the Add assignments pane, assign users. TIP: In the Remove assignments pane, you can remove assigned users. To remove an assignment - 
Select the user and double-click . 
 
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Save the changes. 
