DNS
Provide sender id in sending mails How to Add an SPF Record in Windows 2000 and Windows 2003 DNS
Make sure that your domain dns have spf Record Defined.
to get your right SPF record for hotmail go to this link:
http://www.microsoft.com/mscorp/safety/content/technologies/senderid/wizard/default.aspx
http://www.michaelbrumm.com/spfwindowsdns/
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