If you are having messages been marked as Spam via Hotmail, your network may be blacklist or IP ranges flagged as non-trusted via Microsoft.

A simple solution to guarantee your messages as trusted, is to add an SPF record for your domain(s) then use procedure to submit these details to Microsoft Hotmail

As per: http://www.microsoft.com/mscorp/safety/technologies/senderid/resources.mspx

You are encouraged to e-mail Microsoft after you post your SPF record to the DNS. This will help ensure that your record is automatically included in the SIDF cache and reduce DNS latency.

Send an e-mail message with your domain name in the body of the message (for example, mydomain.com) to senderid@microsoft.com. If you have multiple domain names, please list each domain on a separate line in the body of the message.