There is an option for users on a PHPBB forum to receive an email notification when there has been a reply posted to a specific thread. I usually switch off the option for my own account, because all the extra emails annoy me. So I didn’t know when one of the forums I help administer started having a problem with this.
The users who reported it claimed the notifications had stopped a few days ago. The last change made to the forum was a few weeks ago when I installed an upgrade patch. There didn’t seem to be any explanation for it.
I did a search on Google and there were a couple of reported instances, which all seemed to have something to do with the host server implementing anti-spam measures. It is very likely to be what has occurred in this case. To find a solution I had to find a different way for our forum to send the mail.
It was quite easy in the end. I enabled the option for users to send email to each other from the forum (I’m not sure why this had to be done, but it wouldn’t work otherwise). I changed the mail sending config from ‘mail’ to ‘smpt’ and specified the hosting account username and password. For the mail server I specified ‘localhost’. Did the trick!