What does “relay denied” mean?

Relay denied indicates the server refuses to forward your mail:

What relaying means:

Relaying is forwarding mail to another server. Open relays forward for anyone (now considered abuse). Proper servers only relay for authorized senders/domains.

Why relay is denied:

Wrong server: You connected to a server that does not handle this domain. Authentication missing: Server requires authentication for relay. Anti-spam protection: Server prevents unauthorized relaying.

Typical codes:

550 or 553 with "relay not permitted" or "relaying denied." Enhanced code 5.7.1 (not authorized).

Response:

Verify you are connecting to the correct MX server. Check if authentication is required. Confirm the domain's mail routing.

Relay denied means "not my job." The server does not handle mail for this destination.