When you choose any SaaS based email verification they all give you a existing status of email verified from their database. When you use Real-Time API it should verify the email on the fly every time and give the result instead of pulling the result from the existing database.
BounceKick (Self-Hosted Email Verification Software) has a Real-Time API feature which always does the verification through SMTP check each every email as fresh. This give you a 100% valid result.
Email Checker (www.emailchecker.com) offers an industry-leading Real-Time API that is quick and easy to integrate.
You can sign up for a free API key and account here: https://login.emailchecker.com/?api
You will also get 100 free credits when you sign up for an account so you can carry out some testing:
I recommend using Zerobounce. They have API real-time validation, that can be integrated with every website, and they have a bulk-API validation. The response time for the API is between one second and 70 seconds. On average 96-98% of all domains will return in 1 to 5 seconds. All the major ISP will return in 1 to 3 seconds, which is usually the majority of most email distribution.
Have you tried ZeroBounce? Click here to visit the homepage and get a chance to test some emails, and, even better, make sure to register for an account, you will receive 100 email validations to properly test the service, no credit card required.
The API is fast. It supports up to 50,000 concurrently requests and the average response time is between 2-4 seconds with a customizable timeout parameter.
It's easy to use and has great integrations. Directly import and export to Mailchimp, Constant Contact, AWeber, Hubspot, and many others. Here is the official list.
Very good documentation and support. Here is a place to find everything you need to know if you want to integrate the API into a website or program.
24/7 support. Do not hesitate to contact the customer support for any question
Almost all email verification service providers will support real-time validation through API, but would highly recommend Clearout real-time instant verification API, as it has been constantly optimized for the blazing response time (<300 milliseconds) for free domains like Yahoo, Gmail, AOL etc.,
Also, the API comes with the timeout parameter that helps enhancing the best user experience in case verification takes longer than expected.