People can do searches for email in public directories, previous correspondence and using sites like addresses.com. This searchable directory has the largest databases of email addresses in the world.
Other resources allow people to search active emails if an old email address is known. Sites like People Smart search Internet resources and put together a report on its findings, including email and contact information.
When looking for a business email, networking sites like LinkedIn lets members contact each other and lists an email address and contact information on individual's profiles. Searches can also be done by a person's name or by their company.