This is almost like UniChat!
Good luck! As I've learned, people will find endlessly inventive ways to spam messages into your service x.x
I could help test it/find security loopholes, if you want. I'm not amazing at it, but if I can find it, you can assume that pretty much anyone can.
You might want to add timestamps to the messages, though. If it's been like 5 hours since the last message, you might not want to add to that conversation.