How do I start a fake SMTP server automatically?
Login Items

You can use your account's Login Items to automatically launch an application when logging in.


System Preferences.app > Users and Groups > Login Items


This though will only launch the application. A better solution is to find a GUI-less server and use launchd.

postfix

postfix is an open source SMTP server and it is included with Mac OS X. This is likely to be your best option for setting up a server either to do nothing but accept connections, or to become a full mail server.

launchd

Mac OS X uses launchd for starting services on power on and logging in. You can learn more on the launchd manual page. Apple also has developer documentation for creating launchd job tickets.

To work with launchd, you will need an SMTP server that can be started from the command line.