And probably other Moodle tools, as last weekend yahoo has implemented DMARC sender policy [1] in a way that returns a lot of mail coming from our installations for not complying with that Moodle.org forums [2] has a thread about it and has been suggested that a hot fix could be that instead of send the mail as the user posting the message send the mail with from header with the noreply@ address and set the reply to the current user we already have a lot of bouncing mail from yahoo and in the same thread has been suggested that maybe is a problem related with MDL-42580 [4],

[1] http://www.ietf.org/mail-archive/web/ietf/current/msg87153.html
[2] https://moodle.org/mod/forum/discuss.php?d=257981
[3] https://tracker.moodle.org/browse/MDL-42580