[gexla]

Common problem with Hotmail and a bit of a crapshoot. I assume you have done all the general SPF stuff. There is a postmaster website for Hotmail (guidelines, contact info and other tools) and also a whitelist that you can get on if you are ready to pay out the nose. Google around for it.

One thing you can do is to get an account with an external SMTP service (such as Authsmtp) and configure your email server (such as Postfix) to relay your email through that service. Many people use Gmail for this but Gmail requires unique settings that can make this kind of a pain. This allows you to piggyback on the deliverability of the other service.

If you are sending out newsletters, you might consider going with a specialized service such as Constant Contact.

If you look around, there are more blog posts and information sites that are more detailed. One of them is on hacker news as I type this. Good luck.