To be able to start looking for someone's address, you need to understand how email addresses are composed.
Domains are unique identifiers for the organization that manages the email accounts.
They are covered more extensively in Domain Names . Email addresses are case insensitive. Finding Email Addresses Many people are disappointed that there is not a master list of all the email addresses in the world. There isn't one for much the same reason that there isn't a book that has the work telephone number for every person in the world. To make such a master email directory complete, every organization that provides email service for its members would have to submit all additions or removals to a master list.