This account is provided by MediaWiki's MassMessage extension and delivers messages to users as requested by them. Any user with the 'massmessage' right can send such messages via Special:MassMessage on a local wiki. Messages may also be sent from Meta-Wiki to any wiki.
It is also used by the TranslationNotifications extension to deliver talk page messages based on the translators' notification preferences.
To review past mass message deliveries, please consult the mass message log. The log on Meta includes global message deliveries, while each wiki's local log shows message deliveries local to that wiki.
To quickly see who sent a given message, there is also 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.
Although this user has bot rights, it only marks its edits with the bot flag when leaving messages on user talk pages.