It is recommended that you use a format of mydomain.local for your active directory type stuff, i.e. the LAN side configurations. You then use the same mydomain.com (or whatever internet addy .co.uk, etc) for the internet side of things. At least that is how every text book, and website guide that I have seen for Small Business Server indicate things should be set up.
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One of the best websites for everything Small Business Server:
www.smallbizserver.net