What does “domain not found” mean?

Domain not found indicates DNS resolution failure:

What it means:

The domain part of the email address (after @) cannot be located in DNS. No mail server can be identified to receive the message.

Possible causes:

Domain does not exist: Never registered or registration expired. No MX records: Domain exists but not configured for email. DNS propagation: Recent changes not yet visible. DNS server issues: Temporary lookup failures.

Typical codes:

550 or 553 with "domain not found" or "no such domain" messages. Enhanced code 5.1.2 (bad destination domain).

Response:

Verify the domain spelling. Check if domain has MX records. Suppress address if domain is confirmed invalid.

Domain not found means no harbor exists. The destination port is not on any map.