Adding the following configuration snippet to the web.config file will ensure that System.Net.Mail.SmtpClient is configured to save mails to a local folder. When moving to production, just replace this with the configuration using an smtp server, and you should be good to go!

  <system.net>
    <mailSettings>
      <smtp deliveryMethod="SpecifiedPickupDirectory">
        <specifiedPickupDirectory 
           pickupDirectoryLocation="C:\Temp\MailPickupFolder"/>
      </smtp>
    </mailSettings>
  </system.net>