80 Host Or Domain Name Not Found Type Aaaa Update

For example if there is both an a and an mx for a name but the name server has only the a record cached only the a record will be returned.
Host or domain name not found type aaaa. A dns query for somedomain gov says. If the name server does not have any information on the name the request will be forwarded on. The host name for the record without the domain name.
The ipv6 address the aaaa record points to. We automatically append the domain name. The time to live in seconds.
Your server is trying to use ipv6 when sending the mail. This is the amount of time the record is allowed to be cached by a resolver. Here xyz is the host name of the droplet and not the domain name.
This is generally referred to as subdomain. The records returned may not be complete. If you want postfix to never use ipv6 you can change that in the config file as explained in the postfix postconf manual page.