This account is controlled by the m:MassMessage extension. Any user with the 'massmessage' right can send messages via Special:MassMessage on a local wiki which are delivered using this account. Messages may also be sent via Meta-Wiki.
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.