Trumail - Free email verification API
Hunter - Find all the email addresses related to a domain
Proofy - Quickly validate and verify your email lists.
AnyMailFinder.com - Find anyone's email address
ZeroBounce - Removes invalid emails from your list to prevent email bounces from ruining your deliverability.
Norbert - Norbert is an online tool that helps you to find anyone’s corporate email address.