Im trying to make it so you do not need the .domain when logging in but I am not aware of a way to do this.
Context, Im using a virtualmin panel
If you only have one domain then you set the username_domain in Roundcube config.inc.php file:
// 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 - hostname ($_SERVER['SERVER_NAME'])
// %t - hostname without the first part
// %d - domain (http hostname, $_SERVER['HTTP_HOST'] without the first part)
// %z - IMAP domain (IMAP hostname without the first part)
// For example %n = mail.domain.tld, %t = domain.tld
$config['username_domain'] = '';