What is NXDOMAIN?
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NXDOMAIN is the DNS answer that means the domain flat-out doesn't exist. As far as the public internet is concerned, there is nothing there.
For email, it is a permanent failure, and there's no point retrying. When a sending server looks up the recipient domain's MX (or the A record fallback) and gets NXDOMAIN, it issues a 5xx response and bounces the message immediately. That is the "host not found" error. Common causes are unregistered domains, expired registrations dropped from DNS, and typos like gmai.com for gmail.com.
Compare it with SERVFAIL or a timeout, which are temporary and get retried. The short version: SERVFAIL and timeouts mean retry, NXDOMAIN means bounce. An address on an NXDOMAIN domain will hard bounce on the first send, so suppress it.
One place NXDOMAIN is good news: when you check a blocklist, an NXDOMAIN reply means your IP is not listed.
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