We will be acquiring a new office and will be converting them to our domain, installing a new 2008r2 server which will have AD and 2010 exchange ( I know they shouldn't be on the same) We will just be adding it to our domain and will be connected via cisco asa vpn tunnel. What is the best way to set up the exchange server so the users in that office will authenticate to that AD server and email won't be slow. Our domain currently has 2008 R2 AD and Exchange 2007.

This is seems to be an AD issue and not Exchange. I'd suggest asking in the Active Directory forums for any more clarification. First, I'd highly suggest NOT installing Exchange in the new site. Outlook clients work quite well across VPN tunnels and most organizations are consolidating sites to optimize administration and costs. Second, just don't put Exchange on a Domain Controller, life will be miserable. If you have only one server, install HYPER-V and use 2 virtual machines. For the domain controller and keeping log ons optimize, that's what Active Directory sites are for. Just create a new site in Active Directory Sites and Services, specify the appropriate subnets and that will keep logons local to the remote site and more importantly it will also keep your site from going to the remote domain controller.Ed