What is “MX lookup failed”?

MX lookup failed indicates DNS resolution problems for mail routing:

What it means:

The sending server could not find MX (Mail Exchange) records for the domain. Without MX records, the server does not know where to deliver mail.

Possible causes:

No MX records configured: Domain not set up for email. DNS server issues: Temporary lookup failure. Domain expired: Registration lapsed. DNS propagation: Recent changes not yet visible. Typo in domain: Attempting lookup on non-existent domain.

Typical codes:

550 with "host not found." "MX lookup failed" in error message. 5.4.4 (unable to route).

Response:

Verify domain spelling. Check domain's DNS for MX records. Temporary failures may resolve; permanent ones require address suppression.

MX lookup failure means no map to the destination. The mail system cannot find where to send.