My question is: "Has someone solved this problem before, and how did you do it?"

My goal is automating the testing process. Of course, we have tested by using an Exchange server, but I would like to have a mail server stub that can be used instead of the real mail server

I would imagine the solution would be a script that listens on port 25 for SMTP commands, then records the received data as well as acknowledging these requests with the 250 OK message.

I was going to write such a script, but though it worth asking if someone has already solved this problem.