5.5.0Other or undefined permanent protocol status

A permanent problem occurred with the protocol needed to pass the message to the next system, but no more specific code described it adequately. Do not retry the unchanged send.

The X.5.0 pattern means that something was wrong with the protocol needed to deliver the message to the next hop and that the condition cannot be adequately expressed by another available detail code. In the concrete 5.5.0 code, the leading digit assigns the result to permanent class 5.

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

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.

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.

• 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.

1. 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.
2. 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.
3. 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.
4. Stop unchanged retries; before any new send, confirm a relevant change and check suppression state again.

The accepted Microsoft example from an Exchange Online NDR for Outlook.com/Hotmail recipient lookup reads exactly: “550 5.5.0 Requested action not taken: mailbox unavailable”. This is wording used by that specific product, not the standards definition of X.5.0. The accepted evidence provides neither a provider explanation nor a recommended action, so none is attributed.

550 5.5.0 Requested action not taken: mailbox unavailable

Compare 2.5.0 and 4.5.0, which use the same general protocol detail in the success and temporary-failure classes, respectively. Codes 4.5.1 and 4.5.3 describe more specific temporary protocol conditions—an invalid command and too many recipients; do not assign them without exact evidence.

• 2.5.0Other or undefined protocol status
• 4.5.0Other or undefined temporary protocol status
• 4.5.1Temporarily invalid protocol command
• 4.5.3Too many recipients

The canonical T0 meaning of X.5.0 and its class assignment were verified against the IANA registry and RFC 2034, RFC 3463, and RFC 5248. The exact Microsoft example comes from accepted evidence microsoft-ph02-017. The sources were verified on July 17, 2026; retry, diagnostic, and suppression guidance is presented separately as operational advice.

• iana-smtp-enhanced-status-codesT0 source

  Enumerated Status Codes / X.5.0

• 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

• rfc3463T0 source

  IANA registry reference for X.5.0

• microsoft-ph02-017T1 source

  Common error codes in nondelivery reports table > row 5.5.0; Description cell

Last verified: 17 July 2026