Java based Mail Server DevNull SMTP
I found a simple Java based SMTP Server. DevNull SMTP is a dummy SMTP server that can be used for testing purposes. It helps you see all communication between a client and the server and is very useful if you are trying to find problems with your email server or a client that you wrote.
I use Dumbster for unit testing and as a dummy SMTP server. It allows you to examine email sent to the server, not only discard them.