Setting up external email addresses in Exchange 2010

Hello,

We currently run SBS 2003 server and plan to upgrade to SBS 2011 in the next few months. I am familiarising myself with SBS 2011 and cannot see a way to configure an external email address in Exchange Server 2010. In SBS 2003 we created a new user (the external email address) and then in the 'user's' properties E-mail address tab added the external email address and set it as the primary SMTP address.

I cannot see a similar way of doing this in SBS 2011 / Exchange 2010.

Any advice will be greatly appreciated.

 Mark