From Scarlet Blade Wiki
This page covers the in-game mail system that characters can use to communicate.
You are able to send mail to any character within the same faction. You cannot send mail to a character of the opposing faction. Items sent through the mail have a postage fee. You can send AP items through the mail as long as the item(s) is able to be mailed.
Aeria Games has not yet released information on the in-game mail system.