Reading Microsoft mail responses

Microsoft publishes different evidence for its consumer and organizational mail services. Outlook.com returns SMTP responses for consumer recipients. Exchange Online also produces NDR codes shaped by tenant configuration, connectors, recipients and mail-flow rules.

Always retain the full response. A number such as 5.7.1 is a category, not a complete diagnosis.

Outlook.com temporary responses

Microsoft currently documents these reputation-related 421 responses:

`421 RP-001`

The connecting IP exceeded an allowed rate. Microsoft says the limit is related to IP or domain reputation.

`421 RP-002`

The connecting IP exceeded the rate allowed on the current connection. Microsoft again relates the limit to IP or domain reputation.

`421 RP-003`

The connecting IP exceeded the allowed connection limit, with the limit related to IP or domain reputation.

These are temporary failures. Keep the message queued, reduce the applicable rate or parallel connections, and retry with backoff. Do not convert a temporary failure into an immediate permanent bounce.

Outlook.com permanent policy responses

`550 5.7.515`

The sending domain does not meet Outlook.com’s authentication level for domains sending more than 5,000 messages a day to Outlook.com accounts. Verify passing SPF and DKIM, publish DMARC with at least p=none, and ensure DMARC aligns through SPF or DKIM.

`550 SC-001`

Microsoft associates this rejection with spam-like content or IP/domain reputation. Check the complete response, SNDS data, the sending stream and authentication before changing infrastructure.

`550 SC-002`

Microsoft says the connecting server exhibited namespace mining: attempting to discover valid addresses rather than sending to a legitimate known list. Stop the behaviour and investigate the application or list source.

`550 SC-003`

Microsoft believes the IP is an open proxy or relay. Verify relay controls and host compromise before requesting any review.

`550 SC-004`

Microsoft has blocked the IP after complaints about mail from it. Enrol the IP in JMRP through SNDS, process reports and identify the responsible stream. A generic authentication change does not resolve recipient complaints.

`550 DY-001`

Microsoft identifies the source as dynamic IP space, which Outlook.com generally does not accept for direct unauthenticated SMTP delivery. Use appropriately assigned static sending infrastructure.

`550 DY-002`

Microsoft identifies a likely compromised or infected server or computer. Contain and investigate the system before sending again.

`550 OU-001`

Microsoft points this response to a Spamhaus listing. Follow the listing details and removal process supplied by Spamhaus after correcting the cause.

`550 OU-002`

Microsoft associates this policy response with spam-like content or IP/domain reputation. Use the complete transaction and SNDS evidence to distinguish the cause.

Exchange Online NDRs are different

Do not reuse the Outlook.com interpretations for an organizational Microsoft 365 recipient. Exchange Online’s current reference includes errors caused by:

recipient or group permissions;
transport rules;
relay and connector configuration;
directory and routing errors;
message format and size;
tenant security policy.

For example, Exchange Online 5.7.1 commonly means delivery was not authorized, but its exact cause depends on the text and recipient configuration. It is not evidence by itself of poor public sender reputation.

Use Microsoft’s current NDR table and follow the linked page for the exact enhanced code and text. If the response came from the recipient organization’s tenant, its administrator may be the only party able to change the controlling policy.

Evidence to retain

For any Microsoft case, keep:

the entire SMTP response or NDR;
the hostname that returned it;
sender and recipient domains;
sending IP and EHLO name;
UTC timestamp;
Message-ID;
a fresh copy of the message headers where one was accepted.

This is enough to distinguish configuration, reputation, rate and recipient-policy failures without guessing from a short code.

Primary references

Reviewed against these sources on 27 August 2026.

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