I think you do not have the choice now...
Connect to your database (e.g: phpmyadmin...) and make a backup (if you know how to do that, go here : http://codex.wordpress.org/Backing_Up_Your_Database).
Then, go to the table "wp_options" then select the option name "admin_email", click 'Edit', paste the correct Admin Email then click 'Run' or 'Save'. This will store the correct Admin Email address in WordPress database.
You will be able to go your login page, click 'lost password', this will send a new password to the correct mailbox.