I dealt with something similar. I was creating a large directory of childcare centres in Canada. I had thousands of listings with a url but no email address. I created a Mechanical Turk job to ask turkers to go to website and find an email address. Many came back with email addresses like admin@<<actualURL>>.com. After checking a few, I realized that the turkers were just guessing that admin@ would work and I'd approve their work. I ended up having to double check all the work.