See here https://www.phpbb.com/community/viewtopic.php?f=64&t=2302221

As from some time yesterday GMT I have received any email notifications of my subscribed topics on the phpBB.com forum. However the "Notifications" has still incremented by 0ne as topics have been replied to.

I have sent myself an email from my profile page, which has arrived successfully, so it appears to be a bug in the notifications, not the emailer its self.