One of the nice but little known possibilities of the System.Net.Mail.SmtpClient class is the ability to drop emails into a location on disk. This makes it very easy to test your code without the need of an SMTP server.
Update your app.config or web.config like this:
3: <smtp deliveryMethod="SpecifiedPickupDirectory">
4: <specifiedPickupDirectory pickupDirectoryLocation="c:\mails\"/>
You can also configure this setting on the class itself. Files are dropped on the specified location and can be opened using Outlook Express or Windows Mail.