Method "send()" (in Zend_Feed_Rss and Zend_Feed_Atom) outputs HTTP-headers in format: Content-type: application/rss+xml; charset: UTF-8 but must be in format: Content-Type: application/rss+xml; charset=UTF-8 (note: upper-case letter "T" and "=" without spaces). See 1. http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.17 (Content-Type header description) 2. http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.7 (media-type description)