a few tips for ya if it works great
a) run it thru spokeo.com, its a quick way to check most social networking sites and much more
b) try to send $1 to that email thru paypal, if its valid paypal will show you more info about the person you sent money too (business name and or person name and or contact number)
c) try searching google with just the first part of the email like if their email is
[email protected] try searching for "mrjohndoe" etc, sometimes people may have the same email but with different providers
d) many PI's use trap lines, whereas you would tell the party something you know will make them call and once they dial in the 800 number youd get their phone number (now with trap cell phone type services it dont even have to be 800).
If they created the hotmail just for you etc you might be out of luck, but everything is worth a shot