Hotmail isn't going to tell us how they determine if they think something is spam because that helps spammers get email through.
But Jeroen has pointed out something...
If they return address or the reply-to headers don't match the server, that's two things they reject on.
So use this information.
Sure hotmail would love you to become a Bonded Sender. That's stupid and there are lots of reasons to not buy into it.
If you do buy into it then eventually every large ISP domain out there is going to have it's own program you're going to have to pay for.
Don't let them blackmail you.
We ask our clients to not use our mailserver for outgoing email; we tell them that there are lots of reasons why we may get blocked that are totally beyond our control, and we ask them to use their ISPs servers. Then at least it's their ISPs problem.
For mail that comes from forms created on our servers, we explain to our users that it's safest to have them delivered to mailboxes on our servers.
And we peer with AOL; that doesn't cost anything.
Jeff