Why does my site return a 503 error when using Simple Landing Page?
By default, WordPress already returns a 503 when updating plugins or WordPress core.
When a 503 Service status is set, search engines like Google know that you are working on the page (i.e. its coming soon or maintenance).
Google knows that when the 503 status code is returned to check the page again later. Simple Landing Pages instructs search engines to retry the site again in 1 hour.