I've been able to recieve one and reply. But how do I send a message to another player?

You have to find them and then click their name and then press send message

or have then send you a message and click reply.

Or, if you have them in your friend list, you can send them a message anytime by clicking on their name on your friend list.

before you ask:
No, you will never be able to open an e-mail and put in the name of the one you want to reach (without cheating, that is), you will ALWAYS have to run about and find the person you want to speak...
that's also a part of the game, just like the q-docs thing you asked earlier, Mur wants people to be active and search, and not reward the inactive ones who just click the quest list and then stay offline all week long, seeking the answers

Close due to inactivity.