I found how.
In case anyone else is looking for it.
Changes made in main.inc.php. Which is located in the roundcubemail/config folder.
This removed adding username with @domain.com
$rcmail_config['username_domain'] = '';
add domain in the ''.
example
$rcmail_config['username_domain'] = 'yourdomain.com';
And also remove the server entry
$rcmail_config['mail_domain'] = '';
add domain in the ''
example
$rcmail_config['mail_domain'] = 'yourdomain.com';