yes zhang, it was having correct user name and password but as it was live secondary mail server we could not wait further to rectify the issue and as primary server was working fine we rebuild our secondary server with latest iRedMail server.
But now this is new server setup and all new... now if I go to domain properties in my iRedAdmin-Pro panel and try to assign "Domain quota size or Default quota of new user" to something else it is not allowing. it is just stays as it is without changing anything. no respective logs are also generated to detect the issue here...