NOTE: The Assign mail acceptance and Assign mail rejection assignments are mutually exclusive. You can specify whether to accept or deny the recipient's message.
To adjust mail acceptance 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 mail acceptance task to specify recipients whose messages are accepted. - OR - Select the Assign mail rejection task to specify recipients whose messages are rejected. 
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Select the table containing the recipient from the menu at the top of the form. You have the following options: - 
Mail-enabled distribution groups 
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Dynamic distribution groups 
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Mailboxes 
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Mail users 
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Mail contacts 
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Office 365 groups 
 
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In the Add assignments pane, assign recipients. TIP: In the Remove assignments pane, you can remove assigned recipients. To remove an assignment - 
Select the recipient and double-click . 
 
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Save the changes. 
