Domains don't automatically detect Exchange installations and if it is in a test domain, it's basically segregated from all other domains on your network. Mail routing has to be set up manually, and there is nothing dynamic about it for the most part. You should be just fine connecting it to the internet as long as you don't supply it with the IP address of the Exchange server that is routing Internet mail. You should also be able to set up Live Communications server on your test domain without issues as well.
