I was once trying to develop a SMTP sender lib for PHP. After reading a lot's of RFC's, I found it's really helpful to test my code against some standard SMTP

I was once trying to develop a SMTP sender lib for PHP. After reading a lot's of RFC's, I found it's really helpful to test my code against some standard SMTP server so I can know my code is work as expected. But a real SMTP server is not an option as they all have some kind of filters and server settings that will interfere my test, + In this case, I don't think doing TDD with a real working commercial server (i.e Gmail, Outlook etc) is a good idea. It's not only slow, but also wasting (their resources). That's when a fake SMTP server come in handy. Because not only you can use them to Send a lot's of mail, also those emails will not generate any (I hope) real world side effect.