Problem with automatic domain in username

[ipois0ner]

Hi! There is an option in main.inc.php to automatically add a domain to the end of a username. My host requires this, and I am using this Roundcube installation with two domains. The IMAP is correctly configured for both domains, and works. However, here is my php code for the automatic domain for username:
   // Automatically add this domain to user names for login
// Only for IMAP servers that require full e-mail addresses for login
// Specify an array with 'host' => 'domain' values to support multiple hosts
// Supported replacement variables:
// %h - user's IMAP hostname
// %n - http hostname ($_SERVER['SERVER_NAME'])
// %d - domain (http hostname without the first part)
// %z - IMAP domain (IMAP hostname without the first part)
// For example %n = mail.domain.tld, %d = domain.tld
$rcmail_config['username_domain'] = '%z';

This is not working. I want it to login to the selected imap host by adding the domain to the end. For example, if a user named jack logs in with username 'jack' and selects the host 'imap.ipois0n.com', @ipois0n.com should be automatically added to his username.

My installation URL is access.ipois0n.com

Thanks!
Please help!