If you're looking for a jumping-off point, consider these open source projects.
Rnwood's SmtpServer - hosted on CodePlex. an SMTP server component that can be embedded in application to allow them to receive and process emails using the SMTP protocol. Looks to be fairly active at the moment.
OpenSmtp.Net. It's an SMTP server written in C#, and leverages the System.Web.Mail
namespace. This namespace has been deprecated, though, in favour of System.Net.Mail
. There's been no activity since Feb 2005 on this project.
There may be the skeleton of what you'd want in your product, and perhaps you can enhance the code to use the System.Net.Mail
namespace instead.