NeverBounce - Real-time email verification and cleaning to ensure emails never bounce.
Trumail - Free email verification API
Email List Verify - The Fastest Way to Improve Email List Deliverability and ROI
Proofy - Quickly validate and verify your email lists.
ZeroBounce - Removes invalid emails from your list to prevent email bounces from ruining your deliverability.
TheChecker.co - Accurate email verification at low prices.