user email not updating with Sharepoint

I've found a lot of posts about emails but not one like mine. We have a Windows Server 2008 setup that has been running for a couple of years. We brought in a Sharepoint 2010 server. Everything works fine except for emails. I can add a new user under the Server 2008 Active Directory Users and Computers app and email alerts will work fine. I can't get the email to work for users that were in the server prior to adding the Sharepoint server. Does this make any sense and can anyone help me? Do you need more info?
thank you so much,
jc