The domain name you choose for AD doesn't really matter. By default, SBS
will suggest yourdomain.local. You can use this, or myrealdomain.com or
internal.mydomain.com - you just need to understand the ramifications of
each. For example, if you use myrealdomain.com and don't make some small
adjustments in your AD DNS, your users will not be able to find
www.myrealdomain.com on the Internet, because your server will think it's
entirely authoritative for myrealdomain.com and won't find www. This is not
None of this is relevant when it comes to your Internet mail domain,
anyway - you can run AD using mydomain.local and fluffykittycats.com for
e-mail - the wizard will prompt you for this.
There's nothing particularly different/special about running Exchange as a
'public' mail server - I do suggest you take a look at
but again, the SBS setup wizards (in particular, the CEICW) will help you
set this up.