ZeroBounce - Removes invalid emails from your list to prevent email bounces from ruining your deliverability.
CampaignKit - Email Validation for marketers. Detect invalid and malicious email addresses in your email lists.
clearout.io - Clearout Email Finder & Email Verification Service - Drive sustainable business growth by expanding your reach, lowering hard bounces, and improving deliverability rates.
DataValidation - Email validation and email verification service integrated with MailChimp, MailUp, HubSpot and Zapier.
NeverBounce - Real-time email verification and cleaning to ensure emails never bounce.
Null Bounce - Null Bounce is an email validation tool that helps companies decrease the bounce rate for email campaigns. Also, the service is available as an API, which can be used, for example, on sign-up forms to ensure that user submits a valid email address.