as in private anonymous messages? that is interesting. i think it wouldnt be as fair.
like for example:
"i am england and you are france. i write, of all the messages that you receive, the following:
france, please DMZ EC. to prove that i am the real england, i will move north sea to edi and yorkshire to wales."
in such case, no other player will know about the other moves, and as long as i can prove it, france will be able to believe me. so next time, ill write:
"see, i told you i moved x to y and a to b. so whenever its the real england, i will write, Dear France Sir, so you can recognize my message."
in this case, no one else would know what you have written. if you wrote what i originally did on global only, then someone else can potentially say the same and confuse you (which is the whole point of the game).
if everyone can only write once, everyone will think more about to whom to write and what to write. writing a fake letter to france means you cant communicate with your neighbor - russia - who might be important because he has a unit in armenia. you know?
if we can write unlimited, then the game will just be another gunboat