Once we have the newest update form PWI we will have that function if im not wrong.
Who will be able to use within the faction i don't really know cuz didn't tested it yet :/
i think it was marshal and above but im not 100% about that anymore.
was it added with the chill of luneska patch?
That was the one before the recent one?
If so i think it was before that. It was before the one that added the ressource wars.
Edit:
i just checked
It was added in pwi on April 15th 2014
Also:
Faction Mail has been added.
Faction members with a rank of Marshal or higher can send mail to all members of their Faction for 500 coins. Money and items cannot be attached to Faction Mail.