This guide shows you how to grant send on behalf permissions on a mailbox:
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- Open up the Exchange Management Console.
- Expand Recipient Configuration.
- Click Mailbox.
- On the right hand side double click the mailbox you want to modify.
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- Step 1 -> Right click on the user account.
- Step 2 -> Click on Properties.
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- Step 1 -> Click on the Mail Flow Settings Tab.
- Step 2 -> Click on Delivery Options.
- Step 3 -> Click on Properties.
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- Click the Add button to select a user.
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- Select the user or group from the list and then click on OK.
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- Once you have selected the user it will show as above.
- Click OK.
- Back at the main mailbox properties window, click apply and ok.
Hope it helps.