Create a correct SPF record, and ask if Hotmail (MSN) will add your domain to the 'Sender Policy Framework' (a.k.a. Sender ID Framework)
For more info you can do a search on the forum for 'Hotmail'
you can also read this: http://postmaster.live.com/FAQ.aspx
and this: http://www.microsoft.com/mscorp/safe...d/default.mspx
Please note that it's not only Hotmail that you will have a problem with. Yahoo, and many more do also have something like this.