I have an online browser game that I am working on in my free time with a couple thousand players total. I want to implement either a private messaging system or some chat so that players will be able to interact with each other in a different way other than attacking one another.
One thing I am weary about though is SPAM. If I went with some sort of a global chat, that would need moderators, so I'm more inclined to just implement private messages. With that though I see an issue that someone could SPAM other players and fill their inbox with junk messages (some advertisement in the better case, some malicious links or something like that in the worse case).
Is it a good idea to implement private messages and if so, how do I prevent players from abusing it?