Email delivery with Hotmail can sometimes be finicky. There are a lot of requirements for your email to deliver there, so it is important to make sure you are doing everything you need to to ensure that the emails will deliver.
1) Ensure that the IP is not listed on any major blacklists
The IP address needs to be considered, "clean'. The major and most important blacklist to pay attention to is SpamHaus, but you should also check other blacklists as well and have the IP removed from those. You can use SenderScore to give you a good idea of which blacklists the IP may be listed on.
2) Ensure that SPF and PTR records are set properly
You must have SPF and PTR records set up from the domain you are sending from.
3) Ensure that reverse DNS is set up for your IP address
You must have reverse DNS (rDNS) set up. Reverse DNS can be set up directly within our VPS control panel. It may take between 24-48 hours for reverse DNS entries to propagate, so if rDNS has just been set up, it will take some time.
4) Ensure that the IP address is whitelisted at Hotmail.
It is of paramount importance that the IP address be whitelisted at Hotmail. They request sender information for delivery at their servers, so you need to provide them with a little bit of information. Use this link: Whitelisting form.