Setting the 'From' field to send messages from the shared mailbox
1. Compose a new email. If the From field is not already visible (above the To:.. field) select Options > From within the new email window.
Fig 1. Select 'Other E-mail Address' in the 'From' field
2. Select the From field > Other Email Address... When prompted enter the Display Name or email address of the shared mailbox and select OK. When the From field is selected this should now display the email address of the shared mailbox as an available option.
Fig 2. Send from Other E-mail Address window