Proofy - Quickly validate and verify your email lists.
ZeroBounce - Removes invalid emails from your list to prevent email bounces from ruining your deliverability.
Breadcrumbs Free Email Verifier - Free forever to validate up to 10,000 emails with 1-click
Trumail - Free email verification API
Email Verification Tool - A Google Sheets add-on for bulk email verification
Email List Verify - The Fastest Way to Improve Email List Deliverability and ROI