We had a report that some of students where not getting mail from a particular forum. It was working fine for others though (about a third out of 200-odd got the message ok). On checking the cron logs we discovered lots of messages like this...
Processing user 123456
user 123456 can not see 234567
I tracked down the function that does this 'can not see' check but it has a mass of possible checks. It's just an ordinary News forum in a normal course. Any thoughts appreciated.