An important first step to ensure that email sent from your Domain/IP is not blocked is to check its status on known “blacklists”. For example, you can verify your domain or IP from here: http://mxtoolbox.com/blacklists.aspx
Follow these instructions from Microsoft Support to unblock your IP in Hotmail:
1: Ensure compliance: http://postmaster.live.com/Guidelines.aspx
2: Follow the best practices and FAQs: http://postmaster.live.com/Troubleshooting.aspx
3: Adopt a sender ID and use SPF registration. http://clientes.atlanticadigital.net/knowledgebase/193/Mejore-la-recepcion-y-envio-de-sus-correos-hacia-cuentas-Hotmail-y-Yahoo.html
More details on the SPF record specification are available here: http://www.microsoft.com/SenderID
4 Once the other steps are done, please fill out the Sender Information Form to contact email support: https://support.live.com/eform.aspx?productKey=edfsmsbl3&ct =eformts
5 Once you have submitted the stage 4 form you will receive an email from the Microsoft support postmaster address. Please follow any additional instructions sent by Microsoft.
This information and more details can be found in the PDF document published by Microsoft:
http://download.microsoft.com/download/e/3/3/e3397e7c-17a6-497d-9693-78f80be272fb/enhance_deliver.pdf