The problem that I've experienced with Hotmail is even with proper SPF, if you do not stay below their 1% spam threshold, they reject your IP. The solution tha

The problem that I've experienced with Hotmail is even with proper SPF, if you do not stay below their 1% spam threshold, they reject your IP. The solution that I came to in the past to overcome this, working for a large online dating site, was to build a mail relay system that would monitor the response ratio on each IP, and if it dropped below a certain percentage to switch it off onto another. Worked fairly well, only downside is having to manage a lot of IP addresses on your mailer boxes.