Navigate to [https://www.office.com/?auth=2](https://www.office.com/?auth=2)
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You must have access to Admin portal.
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Go into users
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Click the "Search active users list" field. You are searching for the USER's mailbox that is granting access.
Click the desired USER
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Click "Mail"
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Click "Send on behalf of permissions (0)"
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Click "Add permissions"
Search for the USER who you want to send emails as a delegate for the current mailbox.
Select USER
Click "Add (1)"
Saved means your all set, can take between 5-20 minutes to take affect.