Apility.io - Threat Intelligence Anti Abuse API to find out in advance if your users will be good customers. Learn if your users are on an abuse blacklist and decide how to proceed.
CampaignKit - Email Validation for marketers. Detect invalid and malicious email addresses in your email lists.
TrueMail.io - TrueMail.io is an email checker service.
DataValidation - Email validation and email verification service integrated with MailChimp, MailUp, HubSpot and Zapier.
TheChecker.co - Accurate email verification at low prices.
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.