TL;DR
Null MX: the domain says it accepts no email; do not retry unchanged.
What this code means
X.1.10 marks an associated address invalid because its domain has a null MX. Code 5.1.10 pairs that meaning with permanent class 5.
Provider examples
550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient not found by SMTP address lookup550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient yannja@napa365.com not found by SMTP address lookupTechnical meaning
A null-MX domain declares it does not accept mail. An accepted Microsoft response maps to this code, but its recipient-not-found text is provider practice, not a replacement for the standard meaning.
Delivery status
The current unchanged attempt is permanent. The result is not a reason to infer a fault in the sender or in other domains.
- Class
- Permanent failure
- Retry
- Do not retry unchanged
- Suppression
- Check the full context
Retry decision
Do not retry unchanged. Send only to a corrected, independently verified recipient address.
Suppression decision
Use any hygiene action for the exact address under local policy; do not suppress unrelated domains or recipients.
Common causes
- The recipient address is associated with a domain publishing null MX.
Diagnostic steps
- Confirm the exact recipient, response, and domain DNS state before correcting contact data.
Actions by owner
Sender
- Stop sends to the unchanged address and request a verified alternative.
Sender administrator
- Verify the recipient domain’s null-MX condition and update address data only from a trusted source.
Sources
These sources define what this enhanced status code means, mainly through the IANA registry and related RFCs. When provider examples appear on the page, they come from that provider's published documentation. Follow the links to read the original wording in context.
- SMTP Enhanced Status Codes — IANA registry of enhanced mail system status codes.
- RFC 5248 — A Registry for SMTP Enhanced Mail System Status Codes — Creates and governs the IANA enhanced status code registry.
- RFC 2034 — SMTP Service Extension for Returning Enhanced Error Codes — Defines how SMTP returns enhanced status codes to clients.
- RFC 7505 — A "Null MX" No Service Resource Record — Defines Null MX for domains that intentionally accept no mail.
- RFC 7504 — SMTP 521 and 556 Reply Codes — Defines SMTP reply codes for hosts that do not accept mail.
- NDR error 550 5.1.10 RecipientNotFound — Microsoft Learn article for the RecipientNotFound nondelivery report.
- email-bounce-parser (test fixture corpus) — Open-source bounce-parsing library whose test fixtures are pinned locally as evidence.
Last verified:
