thanks for your response. we intend to use its as independent mail servers for different kind of mail, e.g. member notice mail, member re-active mail, etc. we do not want to use it as mail gateway. As our group not holding one media only, each branch will have their own business, they do not want to centralize into one mail gateway. In our branch, we use them as mail sending machine, they are responsible to send member notice mail , member re-active mail,…etc. One PC for one purpose. The PC is not update, which is old Corei3, 3 or 4 generation, 4G ram, 500-1T hard disk only. We want to use DKIM selector, because we need to use the same domain name.