UProc for Sheets - Boost your sales, marketing and IT tasks from Google Sheets
Verifier - Weed out disposable, non-existent domains or invalid emails.
Sheetlist - Discover free Google Sheets for marketing, finance and more
Email Verification on Google Sheets - An add-on for email verification directly in Google Sheets
Mattermark in Salesforce AppExchange - Automatically enrich leads in Salesforce with Mattermark
Email List Verify - The Fastest Way to Improve Email List Deliverability and ROI