No MX record / Domain not found Email / SMTP Origin: Network Last updated 4 Nov 2026

SMTP: 550 5.1.2 Domain not found / 554 5.4.4 Unable to resolve MX / Host or domain name not found

Mail cannot be routed because the recipient domain has no MX (or A) record, or DNS for it fails.

Exact message you may see 550 5.1.2 Domain not foundDomain not found MXUnable to resolve MXHost or domain name not found. Name service error for name=example.com type=MX554 5.4.4 Unable to route
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Seen on: SMTP Linux

Meaning

When delivering, an MTA queries the MX records of the domain; if none exist it falls back to the A record, and fails if that is missing too. Typos in the domain, expired domains, deleted DNS zones, or MX entries pointing at a CNAME (not allowed) are common.

Common causes

  • Typo in the recipient domain
  • Domain expired or DNS zone removed
  • No MX record published for the domain
  • MX points to a CNAME or to a host with no A record
  • Temporary DNS outage (shows as 4xx)

⚡ Quick fix

  1. Check the domain spelling
  2. dig MX domain.com should return one or more hosts
  3. Fix MX to point to hostnames with A/AAAA records (not CNAME or IP)

Detailed fix by platform

Linux

  1. dig +short MX example.com and then dig +short A mail.example.com for each MX host

How to diagnose

  1. MX — Records returned?
  2. Hosts — MX targets resolve?
  3. Domain — Registered and active?

🔧 Still not fixed?

Many errors look alike. If the steps above didn’t solve it, one of these is probably what you’re facing:

Where this error sits

Origin: Network. Errors that are often confused with or caused by this one (click to open):

421 4.7.0 Try again later (rate limited) — SMTP: 421 4.7.0 Our system has detected an unusual rate of unsolicited mail originating from your IP address. Mail sent from your IP address has been temporarily rate limited (Email / SMTP)421 4.7.0 Try again l…450 4.7.1 Greylisted, please try again later — SMTP: 450 4.7.1 <user@example.com>: Recipient address rejected: Greylisted, see http://postgrey.schweikert.ch/help/example.com.html (Email / SMTP)450 4.7.1 Greylisted,…550 5.1.1 — 550 5.1.1 The email account that you tried to reach does not exist (User unknown) (Email / SMTP)550 5.1.1550 5.7.25 PTR record required — SMTP: 550-5.7.25 The IP address sending this message does not have a PTR record setup, or the corresponding forward DNS entry does not point to the sending IP (Email / SMTP)550 5.7.25 PTR record…554 5.4.14 Hop count exceeded - possible mail loop — SMTP: 554 5.4.14 Hop count exceeded - possible mail loop (Email / SMTP)554 5.4.14 Hop count …DKIM: no key for signature — DKIM: dkim=permerror (no key for signature) / key not found in DNS selector._domainkey (Email / SMTP)DKIM: no key for sign…DMARC: no record found — DMARC: dmarc=none (no DMARC record found for example.com) — bulk sender requirements not met (Email / SMTP)DMARC: no record foundHelo command rejected: need fully-qualified hostname — SMTP: 504 5.5.2 <localhost>: Helo command rejected: need fully-qualified hostname (Email / SMTP)Helo command rejected…No MX rec…

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