Integration with Exhcange 2010 is a little different than 2003. Exchange 2003 uses WebDav as default communication while 2010 switched to webservices. If you enable IMAP for your Exchange server, you can use IMAP to send mail. That is probably the easiest solution.
But there are a lot of possible reasons for why your code is failing. It could be firewalls, exchange access rights and more. What kind of error do you get when trying to send mail and which protocol are you using?
If you want to do more integration (like read mail, list contacts etc) I will recommend using a library from Independentsoft
http://www.independentsoft.de/[
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I've used their components in a few projects, and they are perfect for Exchange integration.