Greetings.
As an "end user" I have no insight to offer but at UKZN we have been experiencing a problem similar to that described by Peter, but worse. Emails are generated by only about one in ten posts by students and none of my posts, as teacher. From the little feedback I have had from the IT people, the problem is widespread (not just my course) and may be with the email system rather than the Moodle 2.6 that we recently commissioned.
Regards, Malcolm Jaros