Basicly heestia just do a check with nslookup, if there is a record for your domain - in your case, this step fails. You could check now on your server, if you get a propper output/recprd if you run nslookup mail.domain.tld - if not, you need to check why.
I put webmail.hestia because my other domain uses webmail.domain. My name server record, i dont know if its right. it’s ns1.second_domain pointing to same machine ip. I have my first domain using ns1.domain to same ip.