Ask HN: Problem getting through Hotmail's spam filter?

Hi guys, last time I got really great advice on how to get through Yahoo's spam filter from this post:

http://news.ycombinator.com/item?id=249886

So now I turn to you for your help again... It seems Hotmail has began to not just filter our Emails as spam, but rejecting them completely. I noticed a lot of Emails we sent to Hotmail accounts (as far as I can tell... they aren't made up), they are being rejected completely and bounced back.

We have reverse DNS, SPF record, and DomainKeys set up. And we are not sending massive Emails at all, just sending registration confirmation Emails and notifications. I was wondering if you guys have any experience with Hotmail and how to get through their spam filter (if it is a spam filter problem...)?

Any advice would be greatly appreciated!