How to check if visitor is entering valid email addresses?

Email Checker Offers an industry-leading Real-Time AP solution that you can integrate at point of signing up that will verify the email address enter by the user in real-time and tell you if the address is good or bad and also give the user the opportunity to correct any misspellings or typos.

You can sign up for a free account and trial key to test the solution for yourself here: https://login.emailchecker.com/?api

Yes, the best solution would be to integrate a real-time API to your landing page (registration form) to validate email addresses as they are submitted to you. This will also allow you the possibility to set certain triggers from responses such as, if the response of the email address entered is invalid , then show visitor message/warning (usually something along the lines of "the email address you entered appears to be invalid" or "please check for misspelling")

The results will as good as the service you choose to use, as far as email validation platforms go I recommend trying ZeroBounce for high accuracy, a wide range of features and competitive prices.