To add another address in the sender address dropdown box:
The system will send an email to that address you have added to confirm that you wish to add that address as a sender address you can mail from. Click on the link in the confirmation email to activate the address.
If you wish to resend a confirmation email, you may do so by clicking on the resend confirmation link on the Sender Address page.
If you wish to remove a sender address, you may do this by using the delete button to the right of the address you wish to remove.