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How to use the XML Sitemap validation tool?
How do Sitemap errors affect your site?
Have you validated your XML against the schema given here: http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd
If yes, what was the result?
If not, what is the URL of your sitemap?
If you don't know how to validate the XML against the schema, use http://www.xmlvalidation.com/
Paste the sitemap-XML there, click on "Validate against external XML schema" and paste the schema after clicking the Validate-button.
This will tell you what's wrong with your XML. If you don't know how to interpret the result, please amend your original question accordingly.
Edit: The error was a missing namesapce-URL in the schemaLocation. The first tag has to look like this:
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.sitemaps.org/schemas/sitemap/0.9 http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd">
Strike the above. Looking at Googles site, their sitemap header seems to be a little longer than yours.
It's on this page: https://www.google.com/webmasters/tools/docs/en/protocol.html
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.sitemaps.org/schemas/sitemap/0.9
http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd">