How do relay errors differ from DNS issues?

Relay errors and DNS issues are fundamentally different:

DNS issues:

Cannot find the server. Domain or hostname does not resolve. No route to destination. Happens before connection.

Relay errors:

Server found and connected. Server refuses to forward mail. Policy or authorization problem. Happens during SMTP conversation.

Relay error causes:

Wrong server for this domain. Authentication required. Anti-relay policies. Server not configured to accept for domain.

Typical error messages:

DNS: "Host not found," "MX lookup failed." Relay: "Relay access denied," "Relaying not permitted."

Resolution approaches:

DNS: Verify domain and records. Relay: Verify server configuration and authorization.

DNS failure is not finding the harbor. Relay denial is the harbor refusing your cargo.