RoundCube webmail is a browser-based multilingual IMAP client with an application-like user interface. It provides full functionality you expect from an email client, including MIME support, address book, folder manipulation, message searching and spell checking.
"^/\?_task\=logout" -> "logout_app https://auth.domain.tld"
PHP_AUTH_*
by HTTP_AUTH_*
$rcmail_config['plugins'] = array('http_authentication');