Starting February 1, 2013, we will require all incoming requests to have a “Host” header which complies with HTTP 1.1 Specifications. This header was not required under HTTP 1.0. IPN and PDT scripts using HTTP 1.0 may start failing with “HTTP/1.1 400 Bad Request” errors after February 1, 2013, which will result in IPN messages not being validated successfully, or PDT scripts not being able to retrieve transaction information.
Your Action Required before February 1, 2013 You will need to update your IPN and/or PDT scripts to use HTTP 1.1, and include the "Host: www.paypal.com
" and "Connection: close" HTTP headers in the IPN and PDT scripts.
And see https://www.x.com/content/bulletin-ipn-and-pdt-scripts-and-http-1-1