Enhanced Status Code 5.1.10: Recipient Address Has Null MX
Enhanced Status Code 5.1.10 means “Recipient Address Has Null MX.” The recipient domain has explicitly declared that it accepts no email. A null MX is a deliberate statement, not a misconfiguration, so no address at that domain will ever receive mail.
550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient not found by SMTP address lookupWhat does 5.1.10 mean?
Enhanced status code 5.1.10 means the recipient address has a null MX. RFC 7505 defines the null MX record as a single dot in place of a hostname, which is the formal way for a domain to declare that it receives no mail at all. Microsoft returns 5.1.10 as recipient not found.
The distinction from an ordinary bad address matters for list hygiene. A 5.1.1 tells you one mailbox does not exist and says nothing about the rest of the domain. A null MX tells you the entire domain is out of scope permanently, so every address at it is invalid and always will be. Domains that only host a website commonly publish one, and it is good practice rather than an error.
This code is also a useful signal about data quality. Null MX domains appearing in a list usually means addresses were collected without validation, or that a typo turned a real domain into a similar one that happens to be website-only. Because the condition is permanent and domain-wide, it is safe to suppress every address at a null MX domain at once rather than one at a time.
How 5.1.10 plays out
5.1.10 rejectionWhere 5.1.10 sits: soft vs hard bounce
| Soft bounce (4xx) | Hard bounce (5xx) | |
|---|---|---|
| Nature | Temporary | Permanent |
| SMTP class | 4xx | 5xx |
| What to do | Let it retry | Suppress the address |
| Recoverable? | Often | No |
| 5.1.10 is | ✓ this code |
Common causes of 5.1.10
- The recipient domain publishes a null MX record and accepts no mail by design
- The domain hosts only a website and was never intended to receive email
- A typo produced a domain that exists but does not handle mail
- A parked or brand-protection domain was used as an email address
- The organisation retired the domain for mail and published a null MX deliberately
- A form accepted an address without validating that the domain can receive mail
How to fix 5.1.10
- Suppress every address at the domain, since the declaration is domain-wide and permanent
- Check for a near-miss typo against a similar domain that does accept mail
- Validate at the point of collection that the domain has a usable mail route
- Do not retry, since a null MX is a deliberate configuration and will not change
- Treat a cluster of these as a signal that a data source needs validation
- Check 5.7.27 for the sender-side equivalent, where your own domain has a null MX