What causes 'Line Too Long' Docomo.ne.jp bounces and how to fix?

Key findings

• RFC Compliance: The error points to a violation of the RFC 5322 standard which specifies a maximum line length of 998 characters for email messages.
• Character Encoding: When using non-ASCII characters, proper content encoding (like quoted-printable or base64) is crucial. Incorrect encoding or lack thereof can lead to lines becoming excessively long during transmission or interpretation.
• MTA Behavior: The issue often stems from the sending Mail Transfer Agent (MTA) failing to properly encode or wrap lines with CR/LF sequences at the required interval, leading to non-compliant messages. This can be an MTA configuration issue.
• Recipient Strictness: Docomo.ne.jp (and other Japanese mobile carriers) are known for strict adherence to email standards and may reject emails that deviate, even slightly.

Key considerations

• Content Length: While the issue is technical, consider whether certain elements in your email, such as very long links or base64 encoded images, might contribute to line length if not handled correctly by the sending system.
• Sending Platform: If using a large email service provider (ESP) like Salesforce Marketing Cloud (SFMC), verify their handling of RFC compliance for line lengths and character encoding. Problems might arise if custom templates or specific content types interfere with the ESP's default processing.
• Email Content Review: Carefully review the raw source of bounced emails to identify where the line length violation occurs. This involves checking headers, HTML, and plain text parts for excessively long lines without proper breaks.
• Testing: Send test emails to Docomo.ne.jp addresses with varying content complexity and encoding to isolate the problematic elements. You can also send to a diagnostic mailbox for analysis, as mentioned in this SmarterTools community discussion.

Key opinions

• Platform Blame: Many marketers using enterprise-level platforms like Salesforce Marketing Cloud (SFMC) assume the platform handles all technical compliance, making this type of bounce particularly confusing.
• Encoding Impact: There's a common concern that the use of non-standard characters, such as Japanese, might indirectly lead to encoding issues and line length problems if not properly converted by the sending system.
• Design vs. Deliverability: Some marketers report that attempts to preserve email design integrity might lead to disabling RFC line length enforcement on MTAs, unknowingly causing deliverability problems with strict receivers.
• Specific Domain Issues: Docomo.ne.jp is frequently cited as a domain with particular strictness, similar to how other domains like Yahoo and Rogers or Orange.fr can pose unique deliverability challenges.

Key considerations

• Template Review: Marketers should review their email templates, especially custom ones or those with complex HTML structures, for anything that might generate excessively long lines, such as deeply nested tables or inline styles.
• Character Set Handling: When sending to international audiences, particularly those using non-Latin alphabets, ensure your sending platform correctly handles character set conversion and encoding to avoid unexpected line length increases.
• ESP Support: If encountering persistent issues with a major ESP, engage their support to investigate how their system handles RFC compliance for email body line lengths and content encoding.
• A/B Testing Content: Consider A/B testing simplified versions of your email content with affected domains to pinpoint specific elements (like very long URLs or large embedded images) that might be contributing to the 'Line Too Long' error.

Key opinions

• MTA Responsibility: Experts universally agree that a 'Line Too Long' bounce is a fundamental technical issue with the sending MTA, which is responsible for ensuring email lines adhere to the specified length limits.
• Encoding is Key: If non-ASCII characters are present, the email must use appropriate content transfer encodings (like quoted-printable or base64). Experts contend that if these encodings are used correctly, 'Line Too Long' errors should not occur unless the system is severely broken.
• Configuration Overrides: Some MTAs allow administrators to disable RFC line length enforcement. While this might prevent internal issues, experts warn it will lead to rejection by strict recipient mail servers, causing bounces.
• Diagnostic Approach: The recommended approach is to capture and examine the raw source of the bounced email. This allows for direct inspection of line lengths and encoding, helping to pinpoint the exact cause of the issue and rule out other deliverability problems, such as being on a blocklist.

Key considerations

• RFC 5322 Adherence: Experts advise that all sending systems must strictly adhere to the 998-character line limit (excluding CRLF) as mandated by RFC 5322. This is a fundamental standard for email interoperability.
• MTA Configuration Audit: If managing your own MTA, conduct an audit to ensure line wrapping and content encoding settings are correctly configured and not overridden in a way that would violate RFCs. If using an ESP, confirm their compliance.
• Debugging Tools: Utilize debugging tools or send copies of emails to diagnostic mailboxes that can display the raw message source, including all headers and body parts, to analyze line lengths precisely.
• Character Set Complexity: Be especially vigilant when dealing with complex character sets (e.g., Japanese, Chinese). While standard encodings should handle them, an underlying issue in the MTA's encoding process can still lead to unexpected bounce messages.

Key findings

• RFC 5322 Standard: This foundational RFC, Internet Message Format, explicitly states that 'no line of characters in a message should be longer than 998 characters' and 'should not be longer than 78 characters' for general interoperability.
• CR/LF Termination: All lines of text in an email message must be terminated by the CRLF (Carriage Return and Line Feed) sequence, as per RFC standards. Failure to insert these breaks at appropriate intervals leads to 'line too long' errors.
• Encoding Requirements: RFCs like RFC 2045 (MIME Part One: Format of Internet Message Bodies) detail the various Content-Transfer-Encodings (e.g., quoted-printable, base64) necessary for transmitting non-ASCII characters or binary data. These encodings include mechanisms to handle line length limits.
• SMTP Compliance: SMTP (Simple Mail Transfer Protocol) servers are expected to enforce these line length limits during the transmission of email data, rejecting messages that do not conform.

Key considerations

• Strict Interpretation: While RFCs provide guidelines, some mail servers (like Docomo.ne.jp) interpret and enforce these rules more strictly than others. Senders must aim for full compliance.
• Automated Line Wrapping: Documentation implies that email sending agents (MTAs, ESPs) should implement automated line wrapping for various content types, particularly when handling complex HTML or long URLs, to prevent lines from exceeding the 998-character limit.
• Impact on Deliverability: Failure to adhere to these foundational standards can directly lead to email rejection, impacting deliverability rates and potentially affecting sender reputation, similar to how DNS TXT record length limits can affect SPF validation.
• Debugging: Diagnostic processes for 'Line Too Long' errors should involve examining the raw message source against RFC specifications to identify exactly which part of the email (headers, body, specific content blocks) is non-compliant.