The title sums this one up with ease. After transitioning / migrating from Microsoft Exchange 2003 to Exchange 2010 some of the first accounts that we tested with were unable to connect to EAS. Errors on the mobile client ranged from server not found/available, to bad user name and password errors. After much research and troubleshooting I came across a few users that were experiencing the exact same issue. You can read more about the issue and the fix here.