How to make registration activation email “to” field use registrant’s first name

Add the following code to your theme’s functions.php file if the plugin provides a filter for modifying email headers:

function custom_user_registration_email_to_field( $email ) {
    $user_id = $email->get_user_id();
    $user = get_userdata( $user_id );
    if ( ! empty( $user->first_name ) ) {
        $email->set_to( $user->first_name . ' <' . $user->user_email . '>' );
    }
    return $email;
}
add_filter( 'user_registration_email', 'custom_user_registration_email_to_field' );