Google’s John Mueller confirmed on Twitter that Google may stop fetching your XML Sitemap files if the URLs within them are invalid. He said it is fine to have URLs that redirect but they should load content. If not, Google may simply start ignoring your Sitemaps. John said “We’d stop fetching sitemap files if the URLs are invalid, but if you’re returning content or redirecting (which is kinda recommended), we’ll keep trying them. It shouldn’t cause problems, since overall sitemap files are only a tiny-tiny part of all URLs fetched from a site.” So make sure the URLs in your XML Sitemap files for the most part return URLs that load content. Forum discussion at Twitter.