How to verify email address entered?

Hello, The mail sebnt from the form to the email adress need to be verified, Only once, how can I do ? 

There is an option to verify email address by enabling this option when the form is on Cards layout.

But, when using Classic layout (All Questions on One Page), I suggest taking advantage of E-mail Validator widget:

https://widgets.jotform.com/widget/e-mail_validator

How-to-Add-a-Widget-to-Your-Form

Let us know if you need further assistance.