curl --verbose 'https://foo.bar.net/bim/bam'
*   Trying 10.10.10.10...
* Connected to 10.10.10.10 (10.10.10.10) port 80 (#0)
* Establish HTTP proxy tunnel to foo.bar.net:443
> CONNECT foo.bar.net:443 HTTP/1.1
> Host: foo.bar.net:443
> User-Agent: curl/7.44.0
> Proxy-Connection: Keep-Alive
>
< HTTP/1.1 503 Service Unavailable
< Cache-Control: no-cache
< X-XSS-Protection: 1
< Connection: close
< Content-Type: text/html; charset=utf-8
< Content-Length: 750
< Pragma: no-cache
< Set-Cookie: frobnicate; path=/; Httponly
<
* Received HTTP code 503 from proxy after CONNECT
* Closing connection 0
curl: (56) Received HTTP code 503 from proxy after CONNECT