I have been trying to get PEAR::mail to successfully deliver emails to hotmail users without being flagged as SPAM and ending up in the junk folder, i have no problems with
I'd suggest modifying the headers you send to match 100% what outlook sends, and see if that solves the problem. Really it's a tough one though, hotmail is known for having a super crappy spam filter, sending lots of legit email to junk, and lots of spam to your inbox.