There are a few issue with Hotmail at the moment as they change their spam system and test out their new version of hotmail.
What exactly is the problem?
It's a weird one.
One of our clients was having serious issues, but once he fixed the headers etc., of his mails his problems have gone away. However another client seems to be doing everything "right" and is having serious delivery issues.
I was just wondering was it an isolated incident more than anything else
New senders to Hotmail are being restricted until they build up a reputation.
They are limiting the delivery of emails that come from new sender IP addresses. At least until they get a handle on the reputation of that new sender.
There are also a few teething problems: Hotmail's antispam measures snuff out legit emails, too | The Register