Hi,
open /config/main.inc.php and go to
// 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
$rcmail_config['username_domain'] = 'mydomain.com';
if the user types: itisme, then at the end his name will be
itisme@mydomaina.com