Send as external domain?

Hi all,

How do I configure Exchange to be able to send as an external domain? I have a closed relay which requires authentication, which works on the user's mailbox but not on external domains we don't own.

There's one email I need to be able to send as, or "on behalf of". When I go to send an email I get this error through Outlook:

"Error 550 5.7.1 Client does not have permissions..."

Under Mailbox delegation I can add recipients but obviously a mailbox not on this server will not show. I added a "Contact" but this didn't show either.

Any ideas?

Thanks