cURL error 6: Could not resolve host: localhost (see https://curl.haxx.se/libcurl/c/libcurl-errors.html) #0 /var/www/html/vendor/guzzlehttp/guzzle/src/Handler/CurlFactory.php(155)

I tried to use this module with Docker4Drupal tool. I tried to set the right settings to this module but don't understand why I get this error
cURL error 6: Could not resolve host: localhost (see https://curl.haxx.se/libcurl/c/libcurl-errors.html) #0 /var/www/html/vendor/guzzlehttp/guzzle/src/Handler/CurlFactory.php(155)

I tried different settings that I use in Private message settings form but anyway I see this error.