Make sure you aren't on any blacklists (http://mxtoolbox.com/blacklists.aspx), and ensure you have a valid SPF record (http://mxtoolbox.com/spf.aspx). I added an SPF record which fixed most of my issues, but turns out I was blacklisted by Hotmail (an entire block of IPs was, I just happened to be in that block). I was able to easily get myself removed from the blacklist though.
Make sure you aren't on any blacklists (http://mxtoolbox.com/blacklists.aspx), and ensure you have a valid SPF record (http://mxtoolbox.com/spf.aspx). I added an SPF record which fixed most of my issues, but turns out I was blacklisted by Hotmail (an entire block of IPs was, I just happened to be in that block). I was able to easily get myself removed from the blacklist though.