I am a Premier Field Engineer and have been with Microsoft for 7 years in the New York area, currently focused on helping a large financial services customer optimize their Microsoft infrastructure. While I work primarily in the Active Directory and Windows OS performance spaces now, I used to focus on Exchange from the time 5.5 was released until about the time Exchange 2003 was widely deployed. Luckily since, Exchange sits on both the OS and leverages the directory that are now my core competencies, I still get to spend plenty of time working with the product.
I also have written some open source tools available at http://activedirectoryutils.codeplex.com. These are primarily focused on troubleshooting Active Directory, but can be helpful to anything dependent on Active Directory replication, authentication, and security. You can also find me at my blog at http://blogs.technet.com/ken_brumfield/.