What is an “unresolvable hostname”?

An unresolvable hostname cannot be translated to an IP:

What it means:

DNS lookup returns no address. The hostname has no A or AAAA record. Connection cannot be established.

In email context:

MX record hostname does not resolve. Sending server cannot locate mail server. Delivery fails with DNS-related error.

Common causes:

Typo in MX record: mail.examlpe.com instead of mail.example.com. Missing A record: MX exists but target has no IP. Deleted or expired: Hostname removed from DNS. Propagation delay: New record not yet visible.

Diagnosis:

Query the specific hostname. Check for A/AAAA records. Verify spelling against intended target.

Unresolvable hostname is a name with no address. The ship exists on paper but cannot be found in any port.