Thank's in advance.

The php ngnix integration already worked fine, but your idea it is goot to know what else is obtained with the manual request, I put the result of the execution:

168.122.170:80/correo-proxy-auth/auth.php
* About to connect() to 192.168.122.170 port 80 (#0)
* Trying 192.168.122.170...
* Connected to 192.168.122.170 (192.168.122.170) port 80 (#0)
> GET /correo-proxy-auth/auth.php HTTP/1.1
> User-Agent: curl/7.29.0
> Accept: */*
> Host:192.168.122.170
> Auth-Method:plain
> Auth-User:ricardo.carrillo
> Auth-pass:r3dh4t
> Auth-Protocol:imap
> Auth-Login-Attempt:1
> Client-IP: 192.168.122.1
>
< HTTP/1.1 200 OK
< Server: nginx/1.6.2
< Date: Thu, 12 Mar 2015 15:46:11 GMT
< Content-Type: text/html
< Transfer-Encoding: chunked
< Connection: keep-alive
< X-Powered-By: PHP/5.4.16
<
Array
(
[USER] => apache
[HOME] => /usr/share/httpd
[FCGI_ROLE] => RESPONDER
[PATH_TRANSLATED] => /usr/share/nginx/html/correo-proxy-auth/auth.php
[QUERY_STRING] =>
[REQUEST_METHOD] => GET
[CONTENT_TYPE] =>
[CONTENT_LENGTH] =>
[SCRIPT_NAME] => /correo-proxy-auth/auth.php
[REQUEST_URI] => /correo-proxy-auth/auth.php
[DOCUMENT_URI] => /correo-proxy-auth/auth.php
[DOCUMENT_ROOT] => /usr/share/nginx/html
[SERVER_PROTOCOL] => HTTP/1.1
[GATEWAY_INTERFACE] => CGI/1.1
[SERVER_SOFTWARE] => nginx/1.6.2
[REMOTE_ADDR] => 192.168.122.170
[REMOTE_PORT] => 39783
[SERVER_ADDR] => 192.168.122.170
[SERVER_PORT] => 80
[SERVER_NAME] => localhost
[REDIRECT_STATUS] => 200
[HTTP_USER_AGENT] => curl/7.29.0
[HTTP_ACCEPT] => */*
[HTTP_HOST] => 192.168.122.170
[HTTP_AUTH_METHOD] => plain
[HTTP_AUTH_USER] => ricardo.carrillo
[HTTP_AUTH_PASS] => r3dh4t
[HTTP_AUTH_PROTOCOL] => imap
[HTTP_AUTH_LOGIN_ATTEMPT] => 1
[HTTP_CLIENT_IP] => 192.168.122.1
[PHP_SELF] => /correo-proxy-auth/auth.php
[REQUEST_TIME_FLOAT] => 1426175171.8848
[REQUEST_TIME] => 1426175171
)
* Connection #0 to host 192.168.122.170 left intact
The response it is OK, so the problem certainly is into the auth logic. (index.php script)