ZeroBounce - Removes invalid emails from your list to prevent email bounces from ruining your deliverability.
Trumail - Free email verification API
Proofy - Quickly validate and verify your email lists.
NeverBounce - Real-time email verification and cleaning to ensure emails never bounce.
Free Email Verifier - The first free and accurate email verification service
Email Verification Tool - A Google Sheets add-on for bulk email verification