This post is focused on those of you who have decided to deploy Exchange in a resource forest.  You’re in for tears.  While the resource forest is technically a supported deployment method for Exchange, I’m going to point out what can go wrong in your Exchange world that will keep your admins up at night.
Let’s start with the definition of a resource forest, just in case you’re not sure.  The resource forest approach means that you have one Active Directory forest where your user accounts live and another Active Directory forest where your application (Exchange, in this case) lives.  You have user accounts in the resource forest that are disabled and then externally associated with the users in the user forest.  This of course, requires a trust between the two forests, which you likely have anyway, right?  Right.
I was one of the engineers on the call documented here from last weekend. This was the Friday night from hell.
I won’t rehash the technical details of what happened. If you want to know the tech piece of it, head to the link. There’s no need to restate it – besides, it would bring back memories that might cause me to draw my own blood and scrawl nasty proverbs on the wall.
