I implemented a mail server as a mirror of an existing one following the instructions in the ISP Config manual. The idea is to have 2 mail servers in case of one is going down. The Dovecot sync seems to work fine but now I'm puzzled about how to set the DNS. My domain name is : mydomain.tld The hostname of the first mail server is : mail1.some-domain.tld and its IP address is 18.104.22.168 The hostname of the second mail server is : mail2.some-domain.tld and its IP address is 22.214.171.124 In the some-domain.tld DNS zone, I set a A and AAAA record for mail1 and mail2. I think that's all that should go there. But what do I add in the mydomain.tld DNS zone?