Verio blames Microsoft and vice versa. Apparently these two giants are unable to communicate and do not respond to either's emails.
My query has been running for weeks, but it sounds like the problem is broader and affects other domains too. According to Verio, their senior technicians are working hard on this, but do not have any estimate about when it might be resolved. It doesn't help things when I get a different person dealing with each email that I send to them. The whole thing must be incredibly wasteful of their resources.
My take on the situation? Don't expect any email you send to Hotmail.com will reliably arrive! It is not just Verio/Hotmail. I've seen the same thing with other email service providers and Hotmail.