Verifier - Weed out disposable, non-existent domains or invalid emails.
Antideo - Email, IP address and phone number validation API service
Email Verification on Google Sheets - An add-on for email verification directly in Google Sheets
Email List Verify - The Fastest Way to Improve Email List Deliverability and ROI
Proofy - Quickly validate and verify your email lists.
Trumail - Free email verification API