Web Forms :: Validating An Email Address?

We are developing an Intranet application that looks up some data for a user through hooks into Active Directory. However, our Network guys don't always enter in a users email address when creating a new user. Therefore, our user database is missing the email address for over half of our users. We have over 10,000 users, therefore asking them to enter 5000+ email addresses into each missing email address field in Active Directory isn't an option. I already know how to validate that an email address is correctly formatted using Regex. What I need is a way to lookup the email address entered in the email address textbox and see if it exists on Outlook/Exchange (or looking up the email address using the First Name/LastName would work). If it does exist then the address is valid, if it is invalid then the user typed in the wrong address. I've tried to do something with CDO, but have had no luck. We use Outlook 2003 and ASP.NET 2.0.