A customer received this message:

Disk Space notification

From unknown user
To: xxx#xxx.de

Your Mailbox at  xxx#xxx.de is 91% full.

First i had the idea, that this simple message is a phishing mail. Then i realized, that the mailbox is really almost full.

So jumped to the "System Messages". I wanted to change the message, that a user can be sure, that the notification is for real and from our server. 

I can't find this message in "System Messages". Where can this message be edited?

Thanks!

Kind regards

Martin