In the most cases, we guess emails. We have a big database so when we know the pattern of a certain domain we know exactly what email would the needed person have. After we guess, we verify the email and then give it to the user. Sometimes our guesses are wrong, but we still show that result.
Pardon me, but will you put contributors to check all the contacts in you database by the date they have been added to the system, or will they check them on random, with a large number of contacts remaining unchecked, may be forever?!