What is a DNS-related bounce?

A DNS-related bounce indicates domain resolution failure:

What DNS does for email:

Translates domain names to IP addresses. Locates MX records for mail routing. Provides SPF, DKIM, and DMARC records.

DNS failure types:

Domain not found: Domain does not exist in DNS. No MX records: Domain exists but no mail servers defined. DNS timeout: Lookup did not complete in time. SERVFAIL: DNS server error.

Bounce implications:

Permanent if domain truly does not exist. Temporary if DNS infrastructure having issues. May affect all addresses at that domain.

Typical codes:

550 with "host not found." 5.4.4 (unable to route). 4.4.3 (temporary routing failure).

DNS is the address book of the internet. Without it, mail has nowhere to go.