Recently one of our clients mail server was getting lots email message from unknown users which was causing some trouble so I decided to route all mail to unknown users (for verification ) to a single mail account. This server use sendmail MTA. Sendmail has option called LUSER_RELAY.
Open Sendmail configuration file sendmail.mc using text editor:
# vi /etc/mail/sendmail.mc
Add following macro to file:
# m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf
# /etc/init.d/sendmail restart
Please note that you can also configure virtual domains but it will take some time to configure sendmail virtual domains.