It appears to happen the most when full message text is sent on thread watches. Checking Hotmail's docos on improving email delivery it suggests that massive amounts of unopened email may trigger email to be failed to be delivered.
I think this is a case for reducing how often the watch thread/new conversation notifications are dispatched, and perhaps introducing image beacons to track notification emails being opened. And if a user is getting a lot of un-opened email notifications, automatically stop sending the damn things.
Note; talking to basic Hotmail support hasn't helped so far.