Here is our situation.
We are running a trial version of Lync 2010 to get our feet wet with the technology. Our internal network consist of a parent domain (parent.com) w/ a child domain (child.com). Neither of these domains are ours from an internet perspective, so we have a third domain that we use for email communication (email.com).
Using the sip of <username>@parent.com works just fine but has to be configured manually for each user as we add them - trying to use one of the preset options fails. Ideally, we would want to use the first option (email address) for the sip, which would mean they would log in using <username>@email.com. I added the email.com domain as one of the usable options in the topology and was able to assign it to a user - however, I initially wasn't able to log on to Lync server because it didn't know where to route the traffic to.
I created a DNS entry that points sip.email.com to the Lync server and that allowed for me to log on to the server BUT it doesn't seem to know about availability (I'm not reporting my calendar status to other users) unless I log in the old way (<username>@parent.com). Is there another DNS entry I need to make to send that information? Is there a better way to do all of this?