/**
 * Change the Register Success message 
 * @param WP_Error	$wp_error The WP_Error object.
 * @param string		Redirect destination URL.
 * 
 * @return WP_Error	$wp_error The WP_Error object.
 */
add_filter( 'wp_login_errors', 'wpdocs_filter_wp_login_errors', 10, 2 );

function wpdocs_filter_wp_login_errors( $errors, $redirect_to ) {
	// Run Only if user registration is a success
	if ( strpos( $_SERVER['REQUEST_URI'], 'checkemail=registered' ) !== false ) {

		// Remove the existing Register success message
		$errors->remove( 'registered' );

		// make the changes to register message
		$errors->add( 'registered', sprintf( __( 'Please check your Spam box if email is not to be found in inbox.' ), wp_login_url() ), 'message' );

		return $errors;
	} else {
		// Return the other message and errors
		return $errors;
	}
}