local account with same name as domain account locking out domain account

I have an issue where if i have a local account that has the same username as the AD account, and the passwords do not match, it will lock out the domain account. This is a recent occurrence. The only thing that has changed lately is the deployment of Exchange 2010 CAS server. Does anyone know why Windows 7 would try to pass local credentials to the domain as if they were domain credentials? Perhaps a new feature of Exchange 201o that tries to do some authentication in the background? This is happening to accounts that have had local accounts on their machines for a couple of years without any issues, but suddenly these local accounts are locking out the domain account. I am hopeful there is something I can change on the server side to prevent this.

Thanks,

Rich