Back in the pen and paper era, we used to be able to send slips of paper as diplomatic mail, addressed to a nation number. How does that work now?
They actually still have pens and paper - weird isn’t it! 
I think it works by you sending an email to Rob, the GM, and he then sends it onto the the relevant palyer(s).
That is certainly how the few diplomatic messages allowed in Gunboat games are sent!
Dean
In a normal team game, you have the email address for all members of your team on each turn report email. For contacting neutrals or the enemy, either go via the fora or email via middle earth.