docker-roundcube-server/roundcube/roundcubemail-1.2.2/plugins/http_authentication/config.inc.php.dist

<?php

// HTTP Basic Authentication Plugin options
// ----------------------------------------
// Default mail host to log-in using user/password from HTTP Authentication.
// This is useful if the users are free to choose arbitrary mail hosts (or
// from a list), but have one host they usually want to log into.
// Unlike $config['default_host'] this must be a string!
$config['http_authentication_host'] = '';