SMTP Message Content and Format Status Codes
X.6 status codes concern message conversion, content availability, UTF-8, and format constraints. Read the full code because this subject includes both successful conversion outcomes and permanent failures.
Message content and format conditions can arise from a required conversion, unavailable fetched content, non-ASCII addressing, client reply capabilities, or a UTF-8 header after DATA. The subject does not tell you whether delivery succeeded or whether content must be changed.
Choose the next check
- Check the full result classSeparate a success-class conversion outcome from a permanent content or format failure before changing the payload.
- Check required conversion and content availabilityIdentify whether a required conversion is unsupported or content could not be obtained.
- Check UTF-8 capabilities and headersInspect the address, client reply capability, and message headers in the transaction context.
Common mistakes
- Treating every X.6 response as a permanent rejection.
- Changing message content without checking whether the response concerns client capabilities or a post-DATA header.
- Ignoring a successful conversion outcome that records a material processing change.
Codes in this category
This inventory contains only reviewed codes assigned to this subject. Open a code to see its evidence, diagnostic steps and owner-specific actions.
- 2.6.4Conversion with data loss performedThe message was delivered, but delivery required a conversion in which some data was lost. This is a successful-result warning, not a rejection.Delivery status2xx · SuccessRecommended actionCheck the full context
- 2.6.8Required UTF-8 reply not permitted by the SMTP clientA reply needed to show the mailbox name would have to contain a UTF-8 string, but the SMTP client does not permit that form of reply. Code 2.6.8 reports this as a success status, not a rejection.Delivery status2xx · SuccessRecommended actionCheck the full context
- 5.6.3Required conversion is not supportedThe message failed permanently because forwarding it required a content conversion that a host in the path could not practically perform. Do not retry the same unchanged attempt.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
- 5.6.6Message content is not availableThe attempt failed permanently because the message content could not be fetched from a remote system. Do not retry the same unchanged attempt.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
- 5.6.7Non-ASCII address is not permitted for the sender or recipientA system permanently rejected a MAIL or RCPT command because the sender or recipient address contained non-ASCII characters that were not permitted in that context. Do not retry the same unchanged attempt.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
- 5.6.8Required UTF-8 reply not permitted by the SMTP clientThe attempt failed permanently because showing the mailbox name required a reply containing a UTF-8 string, but the SMTP client did not permit that form of reply. Do not retry the same unchanged attempt.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
- 5.6.9Message with a UTF-8 header cannot be transferredThe message was permanently rejected after its data had been transmitted because it could not be transferred with its UTF-8 header to one or more recipients. Do not retry the same unchanged attempt.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
Related resources
Guide
- Transactional Email Design
Build MIME/content correctly so conversion and format limits do not fail delivery.
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