The browser saying "this site is not available" is not promising, you should (if you can) enable more verbose logging on the server side to see exactly what is happening. You should also paste both the From
and To
URLs into this useful header analysis webtool and add the details they give you into your question. It will help determine if the error you are getting is a "dns says there is no such URL" or a "server is misconfigured" error, or... whatever :)
Regarding the RewriteCond/Rule you have, as noted in this forum post you need to should escape those periods in your RewriteCond, and make sure that you explicitly call a redirect instruction (assuming you want it permanent vs temp) and may as well specify 'No case' too:
RewriteEngine on
RewriteCond %{HTTP_HOST} (\w+)\.at\.domain\.com [NC]
RewriteRule .* http://domain.com/at/index.php?at=%1 [R=301,NC,L]
Confirm too that just going directly to http://domain.com/at/index.php?at=test
does work.