If you're trying to guess someone's email address, you don't really have a lot of options other than trial and error. However, tech site Digital Inspiration shows you how to use the Gmail tool Rapportive to track down the correct email address of almost anyone.
The process here still requires some trial and error, but it works faster than randomly sending out emails. Start with a list of possible email addresses, and then paste those into a new Gmail message. Rapportive will scour the web to pull up social media profiles for accounts linked to that email. If you get a match, you have the right address.
It's not going to work every time, but if you need to track down someone's email, this could do the trick.
How to Correctly Guess Someone's Email Address [Digital Inspiration]