You can't check that an email is valid in any way other than by sending an email to it and getting a reply. You can check it fits the criteria of an email address - firstname.lastname@example.org - but that does not in any way validate the email.
This is one of the reasons that registrations frequently use a "confirmation link" email - if the address is valid and working (at the time of the receipt) then the user cannot complete registration, and thus can't log in.
But be aware that I can think of two websites providing "throw-away" email addresses for just this purpose, and there are probably many, many more. Just because an email address woprked once, does not mean it will work again!