Link above will take you to it without being registered. The login screen you see on that he page is the login screen to the member mail. It will not work for you to use, you will need to set it up on your site with the email accounts you created within your domain.
I have been using it for years.
Note: Registration is now on, though there is no need to register unless you want to.
If you want to test it out on my test site, I will set up a
testaccount@blind-eddie.com email address if you want.