+ ; MTA-STS
+ ; https://blog.delouw.ch/2018/12/16/using-mta-sts-to-enhance-email-transport-security-and-privacy/
+ ; https://support.google.com/a/answer/9261504
+ _mta-sts${suffix} IN TXT "v=STSv1;id=20190630054629Z"
+ _smtp._tls${suffix} IN TXT "v=TLSRPTv1;rua=mailto:postmaster+mta-sts@immae.eu"
+ mta-sts${suffix} IN A ${myconfig.env.servers.eldiron.ips.main.ip4}
+ ${builtins.concatStringsSep "\n" (map (i: "mta-sts${suffix} IN AAAA ${i}") myconfig.env.servers.eldiron.ips.main.ip6)}
+