What causes “host not found”?

"Host not found" indicates DNS resolution failure:

Common causes:

Domain does not exist: Never registered or registration expired. No MX records: Domain not configured for email. DNS misconfiguration: Records incorrectly set up. Typo in domain: Misspelled domain name.

Less common causes:

DNS propagation delay: Recent changes not yet visible. DNS server issues: Authoritative nameserver problems. Network issues: Cannot reach DNS servers.

Diagnosis steps:

Verify domain spelling. Check domain registration status. Query MX records directly (dig or nslookup). Test from multiple DNS resolvers.

Resolution:

Correct domain if typo. Suppress if domain invalid. Retry if temporary DNS issue.

Host not found means no destination on the map. Verify the domain exists and has mail records.