I have been troubleshooting an ASP.NET application with respect to e-mail notifications going out. One of the most useful things I've run across with respect to this is Dave Wanta's site at www.systemnetmail.com. This site helped provide very in-depth details in a well organized fashion to the System.Net.Mail classes, configurations, and workings. The tip that helped me specifically the most was his explanation of how to capture a log file of the Smtp session – this tip is at http://www.systemnetmail.com/faq/4.10.aspx