Send From another Mailbox (Outlook)
If you wish to send an email so that it’s sent from another mailbox that you have access to you’ll need to add the “From” field to the message window.
In the new mail go to the Options menu and From
This will create the From button
Click on the From button and select the drop down option for “Other Email Addresses…” This will produce the “send from other email address” box.
Select the From button and find the mailbox you wish to send from in the directory, then select OK
Finally select OK again