For general discussion about the MassMessage tool, please post at the Project chat.Edit
This account is part of the MediaWiki software and it is not owned by any user. It performs automatic tasks based on instructions from the MediaWiki software itself. This account is controlled by the MassMessage extension.
To see who actually sent the message, there is a hidden comment in the wikitext source of the text added, indicating who sent the message, what wiki they sent it from, and what input list was used. If you do not wish to receive any messages, add your user talk page to Category:Opted-out of message delivery.