Email Verification on Google Sheets - An add-on for email verification directly in Google Sheets
Proofy - Quickly validate and verify your email lists.
Sheetlist - Discover free Google Sheets for marketing, finance and more
Trumail - Free email verification API
Clearbit Salesforce Prospector - Quickly build targeted lead lists directly in Salesforce
Email List Verify - The Fastest Way to Improve Email List Deliverability and ROI