is_feed() not working online

Hey guys,

I struggle solving this problem for a while now and decided to ask you. I use the following code to redirect my RSS feed links to FeedBurner:

add_action( 'template_redirect', 'feed_redirect' );
function feed_redirect() {
	if ( is_feed() && !preg_match( '/feedburner|feedvalidator/i', $_SERVER['HTTP_USER_AGENT'] ) ){
		header( 'Location: https://feeds.feedburner.com/smaxtra-edition' );
		header( 'HTTP/1.1 302 Temporary Redirect' );
	}
}

The strange thing is that this works just fine in my local testing environment (MAMP) but it does not work online at smaxed.com/feed.

is_feed() seems to return false which is not the way it is supposed to act, right? Any hints? Thanks in advance, I appreciate this!

Best regards,
Stephan

PS. I know that I can also try to use plugins or .htaccess redirects, but I thought using hooks is clean and sufficient.