Sitemaps help search engines to better understand the Joomla! site structure and also help crawlers to easier discover fresh content.
Generating your XML Sitemap for Joomla! has never been easier with RSSeo!
Enable canonicalization first
Before submitting the sitemap to search engines, enable canonicalization to avoid errors in sitemap.
Is this step important?
Canonicalization is active when a permanent redirect exists from mydomain.com to www.mydomain.com (or viceversa), so when a user types in the browser the name of your site without www in the front, he will be redirected to the www version.
If canonicalization is not enabled, both versions of the site will be indexed with AND without www, because search engines don't know which one to pick, decreasing your search engine rank.