TL;DR
A permanent SMTP protocol error occurred with no more specific subcode. Usually points to client or relay misconfiguration rather than recipient list quality. Permanent: do not retry unchanged; fix the protocol exchange before resending to protect deliverability.
What this code means
As a catch-all for permanent protocol problems during handoff to the next system, code 5.5.0 applies registry pattern X.5.0 when no finer detail code fits. The first digit places the result in class 5. The status does not name the affected command, session stage, or root cause, and it does not prove the recipient address is bad. Diagnosis needs the full SMTP response, trace, and returning system context.
Provider examples
550 5.5.0 Requested action not taken: mailbox unavailable550 5.5.0 Requested action not taken: mailbox unavailable (S2017062302).550 5.5.0 Requested action not taken: mailbox unavailable. [#.eop-nam12.prod.protection.outlook.com]Technical meaning
Something was wrong with the protocol needed to deliver the message to the next hop, and no other available detail code can express that condition adequately: that is X.5.0. In the concrete 5.5.0 code, the leading digit assigns the result to permanent class 5.
Delivery status
The leading digit 5 denotes a permanent failure of the current attempt. This is a general code: by itself it identifies neither a particular command, stage, nor cause of the protocol problem, and it does not establish that the recipient address is invalid.
- Class
- Permanent failure
- Retry
- Do not retry unchanged
- Suppression
- Check the full context
Retry decision
Operational guidance: stop automatic and manual retries of the same unchanged send. Consider a new send only after establishing the cause from the full context and confirming a relevant change in the condition, configuration, or handling.
Suppression decision
Operational guidance: do not apply automatic suppression based on code 5.5.0 alone. Inspect the full response, stage, system returning the code, delivery history, and other permanent signals, then decide according to the established cause and applicable policy.
Common causes
- A permanent problem occurred with the protocol required to pass the message to the next system, and the reporting system could not describe it with a more specific available code.
Diagnostic steps
- Inspect the raw SMTP response or delivery report and confirm that the enhanced code is exactly 5.5.0 and that the basic reply is in the 5xx class.
- Correlate the response with the intended message, attempt time, SMTP stage or DSN context, and next system on the path; retain the full response and available logs because the code itself is general.
- Use the retained context to determine whether a more specific protocol status or a confirmed provider-specific condition describes the problem; do not assign a cause from 5.5.0 alone.
- Stop unchanged retries; before any new send, confirm a relevant change and check suppression state again.
Actions by owner
Sender administrator
- Stop retries of the unchanged send and retain the full response, time, stage, and next system for diagnosis.
- Give the collected context to the provider if the cause remains unclear; permit a new send only after a relevant change has been confirmed and suppression has been checked again.
Provider
- If you operate a system involved in the handoff, inspect logs and the available protocol trace for the specified time, stage, and next system; do not derive a specific cause from the code alone.
- Correct the confirmed problem in the managed layer and preserve the exact class and response context; if a more specific protocol status is known, return it instead of general code 5.5.0.
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 3463 — Enhanced Mail System Status Codes — Defines the class/subject/detail model for enhanced status codes.
- Email nondelivery reports and SMTP errors in Exchange Online — Microsoft Learn overview of Exchange Online NDR and SMTP errors.
- email-bounce-parser (test fixture corpus) — Open-source bounce-parsing library whose test fixtures are pinned locally as evidence.
- smtp-codes (community corpus) — Community-maintained reference of provider SMTP responses, pinned locally as evidence.
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