Some email notifications not going out.
 

The PM notifications, new user notifications and new post notifications are not being sent out. I did the diagnostic from within vBulletin and it's says it's working and the test email fires off ok.

However, the server host says there's nothing wrong with the server. We have two vbulletin sites on that server, one is ok, one is not. Was working fine and then just stopped. No mods were added.

Where can I check first?