In Total War, there are two types of communication: Regular Communications and Private Messages.

Regular Communications are primarily done by posting in the forums where the Total War game is taking place.

Below is the standard format for a post (on Giantitp forums):

Nation Name
Defensive Espionage: X

To Recipient 1, Recipient 2... (lowest Defensive Espionage of you and all recipients)
Message text

Regular Communications can be read by anyone with an Espionage higher than the message Defensive Espionage, which is normally contained in the parenthesis. The message's Defensive Espionage is generally the lowest Defensive Espionage of yourself and the message's recipient(s) but certain GMs may decide to have other rulings which would be spelled out in their recruiting thread. Be aware that unless you and those you are talking to have an extreme high Defensive Espionage, your messages will likely be read by someone.

Private Messages are more secure than regular communications, but often have restrictions attached. If the game uses VIPs, a common mechanic is that the only way to have a PM is to have a face to face meeting of VIPs from the respective nations. This uses up the VIP's action for the turn in order to discourage people from using too many PMs. In games without VIPs, there is sometimes a limit on PMs/turn, in order to avoid abuse. However, more commonly in games without VIPs, is is normal to simply tell people not to use too many of them, and keep it on a honor system. There is no standard format for a Private Message. They can either be sent in the format of a regular communication with the [PM] tag, or you can send them directly to the player via the forums private message function. If it is sent through the forum's message system, it is highly advised to forward all of the messages to the GM so they can stay up to date on what is happening in their game.