We can verify the existence of email address using some Linux commands.
For example here we are verifying the mail-id firstname.lastname@example.org
Step1: Find mail server of domain example.com.
#dig mx example.com
Let the mail server of example.com as mail.example.com.
Step2: Connect to the mail server using telnet command as follows.
telnet mail.example.com 25
Connected to mail.example.com.
Escape character is ^]
220 mail.example.com ESMTP
mail from: <email@example.com>
250 2.1.0 Ok
rcpt to: <firstname.lastname@example.org>
550 5.1.1 <email@example.com>: Recipient address rejected: User unknown in virtual alias table
221 2.0.0 Bye
Please note that we could not assure 100% result in this method as some mail providers like hotmail.com, live.com are blocking these kinds of email verification process.