[Plugin: Google XML Sitemaps] Why i am getting error in webmasters tool

Dear friends can any one guide me why i am getting error in sitemap at webmasters tool. i am getting following error message.

“We encountered an error while trying to access your Sitemap. Please ensure your Sitemap follows our guidelines and can be accessed at the location you provided and then resubmit.”

if i check my sitemap.xml fiole from our website it is showing no error. our site is http://www.theindiapost.com/sitemap.xml

can any one guide me how can we remove error for webmasters tool. Some time webmasters tool show no error in sitemap.xml

http://wordpress.org/extend/plugins/google-sitemap-generator/