Hotmail email accounts aren't getting my email from my Windows Server

I have generate a PHP Send Mail form to send an email out to all of my customers.  The script pulls the email address from my database and sends them via a plain Windows server.  Only accounts that have Hot Mail accounts aren't receiving my emails but everyone else is getting them.  I checked to see if the Hot Mail account has delete spam on, but it has the plain setitings.  I checked to see if it is going to my Spam Folder or Junk Folder and nothing.  Why is this happening>