My Moodle 2.2.4 is running on W2008SP2, IIS 7.5, MySQL and PHP all up to date. Cron is running dependably, and email messages coming from forum subscription and other cron-controlled stuff do make it to their destinations...occasionally, but not dependably. I have one teacher who has complained consistently that she does not receive all email notifications of quiz completions. Some arrive, some don't, and we can see no obvious pattern to the problem. Our SMTP traffic is forwarded on to Microsoft's Live platform for distribution (our institutional email is hosted there), and I have no doubt that such might be part of the problem...but one opened trouble ticket with them went nowhere.
I am not asking for a fix. I'm asking for a list of things I can check to help with trouble-shooting this problem. "Broke" and "Fixed" is something I can wrap my head around. "Sporadic" in this context is way outside my comfort zone!
If there are threads you know of here which might be related, by all means link 'em in. Searches of the forums have been quite unsatisfying.
Thanks in advance.