I think the server has crashed again; I've not received notifications to threads recently, even though they've had replies.
It also appears that the server will 'blacklist' a user's email address if it gets too many 'unavailable' bounce messages; that's what happened to me a while back, when our corporate email addresses changed (they forwarded the old addresses, but sent an 'address changed' response each time). OM's server took these to be a Bad Sign, and stopped sending me responses completely.