smtp4dev - the fake SMTP email server for development and testing.
A dummy SMTP server for Windows, Linux, Mac OS-X (and maybe elsewhere where .NET Core is available). Lets you test your application without spamming your real customers and without needing to set up a complicated real email server with a special configuration. Messages received in smtp4dev can be viewed and inspected.
If you're looking for the older v2 Windows GUI version. Grab it here.