Send from different email addresses - how to do this?

I have configured a clients Exchange 2010 server (running under SBS 2011) to receive emails for other domains. They can receive emails fine to the domains.

The problem I have though is that the client needs to be able to send emails as those other email addresses.

I.e. From within Outlook they need to be able to send an email so that the outgoing email address is one of the other domains.

Is this possible? It seems such an obvious thing to want to do but I can't find any way of doing it.

Thanks

I don't work for Microsoft - I just use their products.