In Contact Form 7, if you hook into wpcf7_mail_sent, then  the $_POST variable is empty, so you cannot use it to do stuff with it. You are too late. However, there is a way to get around this: you have to use Contact Form 7 built-in functions to get the posted data.

$submission = WPCF7_Submission::get_instance();
if ( $submission ) {
    $form_data = $submission->get_posted_data();

    // now $form_data contains the form values,
    // so you can do, for example $form_data['email'], instead of $_POST['email']

}else{
    // we got nothing; something went wrong
}

That’s it.