Thanks for your feedback.
I was curious what you did, because it might be something that could be added in the troubleshooting instructions, in the plugin’s manual. However I am not sure how the SMTP (SendMail Transfer Protocol) settings are relevant to IPNs.
If you are running the plugin in your local network, instead of a public-facing server, then you can indeed configure your modem/router to forward incoming IPNs to your local machine. However, the settings section of your router that would be relevant would be the NAT (Network Address Translation) settings, or port forwarding settings. These can go by other names; for example in Thomson routers, look under “Games and Applications Sharing”.