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This is a topic we covered in the past, but I had the opportunity to remind you all, that Google prefers you use a server status code of 503, when your site is going down for maintenance.
Googler, JohnMu, reminds us in a Google Groups thread that the best way to let Google know your site is temporarily offline is to return a 503 status code. A 503 is known as a "service unavailable" and GoogleBot and other search engines, should take that to mean that your site will be back later and this is only a temporary issue.
Returning a status code of 200, which means everything is okay and showing zero content, can hurt you. So it is best to use those server response codes, when possible, to communicate to the search spiders, what is really going on with your site.
John explains:
Optimally, all such "the server is down" URLs should return result code 503 (service unavailable). Doing that also prevents search engines from crawling and indexing the error pages :-). Sometimes I'm surprised at how many large sites forget to do this...
Forum discussion at Google Groups.
