Hotmail used to require a couple steps to get your sending server IP address whitelisted such as filling out a form with domain name + ip address and then sending email to a special tech support address, but I think they now rely more on Return Path certification (
http://returnpath.com/solutions/email-d ... ification/) which can be very expensive. You can still try though because a form is still at
https://support.microsoft.com/en-us/get ... 1424800188 (found via postmaster tools
http://mail.live.com/mail/postmaster.aspx)
If it's just a handful of known users I think you can get them delivered if the user themselves add you to their own whitelist/contacts list. Some quick instructions for several providers at
http://www.whatcounts.com/how-to-whitelist-emails/