5.1.10Recipient address points to a null-MX domain

The recipient address is associated with a domain marked by a null MX as not accepting mail. This is a permanent failure, so do not retry delivery to the same unchanged address.

The X.1.10 pattern means that the associated address is marked as invalid using a null MX. The concrete 5.1.10 code combines that meaning with permanent class 5; the accepted Microsoft example maps to it by the exact enhanced code, but its recipient-not-found text is provider practice rather than the standards definition.

The leading digit 5 denotes a permanent failure. The unchanged attempt should not be retried; a new send is justified only after the address has been reliably corrected or a change in the domain's state has been confirmed.

Class

Permanent failure

Retry

Do not retry unchanged

Suppression

Check the full context

Operational guidance: stop automatic and manual retries to the same unchanged address. Consider a new attempt only after a confirmed address correction or a fresh verification of the domain's MX state.

Operational guidance: inspect the full response, exact address, and MX result before deciding suppression. Do not blanket-suppress other addresses or domains from a single 5.1.10 result; handle a confirmed address under local policy.

• The domain in the recipient address publishes a null MX, indicating that it does not accept mail.
• Sender data contains an incorrect recipient domain that leads to a null MX.

1. Inspect the raw SMTP response or delivery report and confirm the exact 5.1.10 code and a basic reply in the 5xx class.
2. Compare the full address, especially its domain part, with a trusted source value; do not guess the correct address.
3. Check the MX records for the exact domain and confirm that the result actually denotes a null MX; retain that result with the full response for the handling decision.

The only accepted provider example comes from Microsoft 365 / Exchange Online hybrid: “550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient not found by SMTP address lookup”. It shows that provider's exact use of the code but does not replace the standards definition of null MX.

550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient not found by SMTP address lookup

Compare 2.1.5, which confirms a valid address; temporary 4.1.1 and 4.1.8, which concern the recipient mailbox and the sender system or domain, respectively; and permanent 5.1.1, which means that the recipient mailbox does not exist.

• 2.1.5Destination address is valid
• 4.1.1Temporary destination mailbox address problem
• 4.1.8Temporary sender system address problem
• 5.1.1Recipient mailbox does not exist

The X.1.10 meaning and its permanent class were verified against the IANA registry and RFC 2034, RFC 5248, RFC 7504, and RFC 7505; the exact Microsoft example comes from accepted evidence microsoft-ph02-037. The sources were verified on July 17, 2026. Retry, suppression, diagnostic, and owner-action guidance is operational and distinct from standards and provider facts.

• iana-smtp-enhanced-status-codesT0 source

  Enumerated Status Codes / X.1.10

• rfc5248T0 source

  Section 2.1: registry fields and non-exclusive Associated Basic Status Code

• rfc2034T0 source

  Section 4: enhanced status class agrees with SMTP reply class

• rfc7505T0 source

  IANA registry reference for X.1.10

• rfc7504T0 source

  IANA registry reference for X.1.10

• microsoft-ph02-037T1 source

  Problem > The NDR contains the following error code

Last verified: 17 July 2026