NeverBounce - Real-time email verification and cleaning to ensure emails never bounce.

Antideo Email Validator - Antideo Email Validation plugin for WordPress ensures that you get to validate emails in real time

Email List Verify - The Fastest Way to Improve Email List Deliverability and ROI

CampaignKit - Email Validation for marketers. Detect invalid and malicious email addresses in your email lists.

DeBounce - Email Validation, Email Checker, Data Enrichment and Appending Tool. Using DeBounce remove invalid, disposable, spam-trap, syntax and deactivated emails.

Sender Policy Framework - This is a diagnostic tool that acts as a Sender Policy Framework (SPF) record lookup