Sending notes to other computers on local network may be a cool feature. But not all computers are on all the time. And not obviously PNotes.NET is running on another computer just now. In this case sending will fail and your message will be lost. In order to deal with such situation you may install (or ask you system administrator to do that) PNExchangeService program. Once installed and started, the program will run all the time in background and will start automatically after Windows restart (because it is actually Windows service).
All you have to do - allow storing undelivered notes on server and enter proper IP address and port. Since now undelivered notes will be stored on server and resend to recipient when his/her computer/PNotes.NET will wake up.