Sender Policy Framework

My message didn't arrive at Hotmail, not even in the spam folder. Why not?

Hotmail uses SenderID as an input into their reputation system. Messages that Pass or Fail their SenderID checks get a different reputation. The first thing to check is that your SPF record is correct and current.

Once you are certain of your record, you might try sending via a different provider. It appears that their reputation system uses a combination of the domain reputation and the reputation of the sending IP address. This is discussed in these mailing list threads: [1], [2].