Today I’ve released netDumbster (http://netdumbster.codeplex.com/)
netDumbster is a .Net Fake SMTP Server clone of the popular Dumbster (http://quintanasoft.com/dumbster/)
netDumbster is based on the API of nDumbster (http://ndumbster.sourceforge.net/default.html) and the nice C# Email Server (CSES) writen by Eric Daugherty.
Hope it helps!!!
Update: netDumbster source code was moved to a GitHub repo: https://github.com/cmendible/netDumbster