The existence of an ingame chat proves, that it is possible to write messages on one account and read it on another.
The existence of a mail system proves, that it is possible to receive messages personally.
The existence of system mails proves, that it is possible to include custom texts in those messages.
For over seven years, a feature to write ingame mail messages to other members has been asked for again and again. Other multiplayer games are able to do this (and you would even have trouble to find one, where it is not possible). This forum is able to do this.
If this is about moderation, limit it to guild members at least.
I’ll take this over the next dozen “improvements” and “more contents” combined.