According to several comments, this means that our mail is currently
blacklisted by Yahoo!, so it's sent in the "Bulk" folder for users who have it
enabled (if not to spam?). WMF needs to send a "Bulk sender application for
whitelisting" http://help.yahoo.com/kb/index?page=content&id=SLN9042 but first
their guidelines have to be respected:http://help.yahoo.com/kb/index?y=PROD_MAIL_ML&page=content&id=SLN3435"
see more details on Bugzilla