This account is controlled by the m:MassMessage extension. Any user with the 'massmessage' right can send messages via Special:MassMessage which are delivered using this account. Messages may also be sent via Meta-Wiki.
A hidden comment in the wikitext source of the text added indicates who actually 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:Αποεπιλογή παράδοσης μηνυμάτων.
Please be cognizant of the fact that MassMessage, as its core, does little more than automate what any user could do with browser tabs without even logging in.