Well an obvious solution is that you send a test mail to that email address and if your message doesn't bounce, it is safe to assume* that the address is real.
Some web domains may have configured a catch-all email address meaning that messages addressed to a non-existent mailbox will not be returned to the sender but in most cases, such email messages will bounce.
Ping an Email Address to Validate it!
When you send an email to someone, the message goes to an SMTP server which then looks for the MX (Mail Exchange) records of the email recipient's domain.
For instance, when you send an email to firstname.lastname@example.org, the mail server will try to find the MX records for the gmail.com domain. If the records exist, the next step would be to determine whether that email username (hello in our example) is present or not.
Using a similar logic, we can verify an email address from the computer without actually sending a test message.