I try to contact a certain Minecraft player (Java) but only knows the name of the player.
Is there any way to write to other players?
To my knowledge, only if they are on the same server (network).
I'm not sure.
But maybe. 5zig-Mod.
In Vanilla Minecraft? No. Only if you are on the same server at the same time.
But there are some clients and mods that have their own chat system. If you and he / she use the same client, you can usually find yourself chatting via the username. Such clients are e.g. Labymod, BAC.
