can I setup 2 machine running ISPConfig 3.x and both of them can redundant each other? e.g. server A got domain abc.com (IP 18.104.22.168)and server B got 123.com (IP 22.214.171.124) and running as mail server. If server A is down, all abc.com will forward to 126.96.36.199 and temp store in server B, when Server A up again, the mail will send back to Server A. Same when server B is down. I know this also need DNS MX record to help. I already setup MX 10 point to 188.8.131.52 and MX 20 point to 184.108.40.206. the remaining should be how can postfix receive the emails if there's no such domain record? Is there any way I can set in ISPConfig? or I need to amend relay domain in main.cf ?