smtp4dev is a smtp server designed for use when developing or testing software that sends emails.

The application acts as a smtp server to receive messages being sent, but does not deliver those messages to their intended recipients. Instead the application provides a list of the messages that have been received and allows the user to view them and inspect their structure and source.

smtp4dev is similar to (and was inspired by) PaperCut but provides more features such as SSL/TLS, error simulation, authentication and message searching. I wrote it as an exercise to learn how to write a multi threaded socket server in .NET and to learn more about the SMTP protocol.

The SMTP server component used by the application is called Rnwood.SmtpServer, and has been designed to it can be reused by other applications.