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Hello I'm a seo newbie and some help from the community here would be greatly appreciated. I have submitted the sitemap of my website in google webmasters tools and now I got this warning: "When we tested a sample of the URLs from your Sitemap, we found that some URLs were not accessible to Googlebot due to an HTTP status error. All accessible URLs will still be submitted." How do I fix this? What should I do? Many thanks in advance.