Exchange 2000

I have a client that has 4 email domains configured on his Exchange server. The president also uses Internet email (Outlook) to grab mail from another company. He uses the “From” field to choose which email address he respondes with.
He can’t usehis internet email account in the “From” field. Outlook informs him that he does not have permission to send w/ that email address. For now I have set him up w/ 2 different email profiles and have the second dump to a .pst. Is there a way I can configure this scenario to work. I would like 1 email profile, 1 exchange box, but the ability to answer from any of the 5 email domains?