What causes 'Line Too Long' Docomo.ne.jp bounces and how to fix?
Matthew Whittaker
Co-founder & CTO, Suped
Published 19 Jun 2025
Updated 15 Aug 2025
6 min read
Receiving a 'Line Too Long' bounce message, especially when sending to a specific domain like Docomo.ne.jp, can be a puzzling and frustrating experience. It suggests a technical issue with your email's structure, rather than its content or recipient engagement.
This specific error often points to a violation of email formatting standards defined in RFC 5322. Essentially, a single line in your email's raw source code exceeds the universally accepted character limit, which is typically 998 characters, excluding the carriage return and line feed.
The challenge lies in identifying exactly which part of your email is causing the problem and then implementing the correct fix. It requires a closer look at your email client or sending platform's configuration, your Mail Transfer Agent (MTA) settings, and how your email content is encoded before it even leaves your system.
What 'Line Too Long' means for email delivery
The 'Line Too Long' message, typically presented as something like 5.0.0 (undefined status) Line Too Long, signifies that your email's data stream contains a line of characters longer than the allowed limit. This isn't about the total size of your email, but rather the length of any individual line within its raw format.
This violation often stems from excessively long URLs, deeply nested inline styles in HTML, or improperly handled Base64-encoded images. Mail servers, especially those belonging to strict Internet Service Providers (ISPs) like Docomo.ne.jp, enforce these limits to prevent malformed messages or potential spam. You can find more details on this error in discussions about bounce error messages.
It's a compliance issue with how the email is structured at a very fundamental level. While some less stringent mail servers might attempt to reformat or simply truncate overly long lines, Docomo.ne.jp is known for its strict policies, often rejecting non-compliant messages outright. This means adhering to these technical standards is crucial for successful delivery.
RFC 5322 line length
Hard limit: No line of characters should exceed 998 octets (characters).
Recommended limit: It's advisable for lines to be no longer than 78 octets.
Common causes
Email client settings: Some clients, like Microsoft Outlook, can cause issues when converting Rich Text to HTML.
MTA configuration: The Mail Transfer Agent might not be correctly wrapping lines for RFC compliance.
Content encoding: Improper handling of quoted-printable or Base64 can lead to excessively long lines.
Malformed HTML: Very long inline CSS or unsegmented text within HTML email templates.
Common culprits behind the issue
The culprit for 'Line Too Long' errors often lies in the sending environment. Email clients, particularly older versions or specific configurations, can contribute to this. For example, Outlook is frequently cited as a source of these issues, especially when it converts messages from Rich Text format, which doesn't always handle line breaks optimally.
Another significant factor is how your email's content, especially non-ASCII characters, is encoded. For languages like Japanese, it's essential to use content transfer encodings such as quoted-printable or Base64. These encodings are designed to handle line breaks automatically. If you're still getting 'Line Too Long' errors despite using these, it indicates a more fundamental issue with how your email is being generated or transported.
Finally, the Mail Transfer Agent (MTA) itself can be misconfigured. Some MTAs might have settings that override or disable RFC line length enforcement. While this might seem convenient, it's a risky approach, as it can lead to deliverability issues with other strict recipient domains, potentially leading to your IP being added to a blocklist or blacklist.
Typical email sending
Email clients or sending platforms may prioritize convenience or visual fidelity over strict RFC compliance, sometimes leading to longer lines of code or content.
Some Mail Transfer Agents (MTAs) might have default settings that are lenient with line length, or they might be configured to ignore these warnings.
Docomo.ne.jp specific requirements
Docomo.ne.jp, a major Japanese mobile carrier, has highly stringent email receiving policies. They often reject emails that do not conform precisely to RFC standards, including line length.
Japanese cell phone users can block emails from PCs or only accept specific addresses, adding another layer of complexity to deliverability.
Strategies to resolve 'Line Too Long' bounces
Resolving 'Line Too Long' bounces begins with examining your sending environment. If you're using an email client like Outlook, check its message format settings. Ensure you are sending in HTML or Plain Text format rather than Rich Text, and verify that it's configured to wrap lines correctly. Sometimes, simply changing this setting can resolve the issue.
For those managing their own Mail Transfer Agents (MTAs), review your MTA's configuration. While it might be tempting to disable RFC line length enforcement, this is generally ill-advised as it can create broader deliverability problems. Instead, ensure your MTA is configured to correctly wrap lines according to RFC standards, even for complex content like Base64-encoded attachments. Improving technical email deliverability involves strict adherence.
If you are sending marketing or transactional emails via an Email Service Provider (ESP), inspect your email templates. Overly complex HTML structures, very long inline CSS, or unsegmented strings of text can be the culprits. Most reputable ESPs handle line wrapping automatically, but it's wise to test your emails by sending them to a hidden address and examining the raw source to identify any lines exceeding the limit. This level of scrutiny can help in fixing max message size exceeded errors.
Example of an improperly formatted email line causing bounceemail-raw
Received: from mail.example.com (mail.example.com [192.0.2.1])
by recipient.docomo.ne.jp with ESMTP id ABCDEFGH for <recipient@docomo.ne.jp>;
Mon, 1 Jan 2024 12:34:56 +0900 (JST)
From: sender@example.com
To: recipient@docomo.ne.jp
Subject: Test Email
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
This is a very long line of text that exceeds the RFC 5322 recommended limit of 78 characters and approaches the absolute limit of 998 characters. Such a line often occurs due to unformatted plaintext or improperly encoded content, leading to 'Line Too Long' bounces, especially when sending to strict receivers like Docomo.ne.jp. This particular line is designed to illustrate how easily a line can become excessively long if not properly wrapped or encoded, leading to deliverability issues and rejection by compliant mail servers. This can include URLs, embedded styling, or raw data streams within the email body. Check your email's raw source to pinpoint the exact line causing the problem and consider how your sending system handles line breaks and encoding for optimal deliverability. Ensure compliance with RFC standards for all outbound email traffic to avoid such technical bounces in the future. Proper email hygiene involves continuous monitoring and adherence to these standards to maintain a good sender reputation and ensure your messages reach the intended recipients without interruption, thereby preventing messages from being treated as spam. It's crucial for your email infrastructure to handle these details correctly.
Ensuring robust email deliverability
The 'Line Too Long' bounce from Docomo.ne.jp is a clear indicator that your emails are not fully compliant with RFC 5322 standards. While it may seem like a minor technicality, addressing this issue is vital for ensuring your messages reach their intended recipients, particularly on highly regulated networks.
Effective resolution involves a thorough review of your email client's settings, your Mail Transfer Agent's configuration, and the inherent structure and encoding of your email content. Proactive email deliverability testing and debugging, including inspecting raw email sources, will help pinpoint and rectify the precise cause of these bounces.
By ensuring your emails strictly adhere to RFC standards, you not only eliminate 'Line Too Long' bounces but also significantly improve your overall deliverability, reduce the chances of your emails being flagged as spam, and maintain a positive sender reputation. This fundamental compliance is a cornerstone of robust email deliverability in the long run.
Views from the trenches
Best practices
Always ensure your emails, particularly their headers and body, respect the 998-character line length limit specified in RFC 5322.
For non-ASCII characters (like Japanese), use appropriate content encodings such as quoted-printable or Base64 for proper line handling.
Before large campaigns, send test emails to various providers, including problematic ones like Docomo.ne.jp, to proactively check for bounces.
Common pitfalls
Dismissing 'line too long' as a minor issue can lead to widespread deliverability problems, especially with strict ISPs.
Turning off line length enforcement on your MTA might seem like a quick fix, but it can negatively impact deliverability to other recipients.
Overly complex or poorly coded HTML email templates can create very long, unsegmented lines, leading to bounce errors.
Expert tips
Analyze the raw source of bounced emails to pinpoint the exact lines exceeding the length limit and identify the specific content.
Verify that the email client or platform used for sending is not introducing improper formatting or excessive line lengths.
Examine your Mail Transfer Agent (MTA) logs for more detailed error messages or insights into how the email was processed before bouncing.
Expert view
Expert from Email Geeks says: The 'Line Too Long' error often points to an underlying issue with how the sending MTA is handling email content, specifically regarding RFC 5322's line length limits. It's a configuration issue or a bug in how the email is formatted before sending.
2020-12-09 - Email Geeks
Expert view
Expert from Email Geeks says: When dealing with non-ASCII characters, proper encoding like quoted-printable or Base64 is essential. If these are correctly applied, line length errors are usually indicative of a more severe issue with the email generation process, not just the content itself.
Docomo.ne.jp, can be a puzzling and frustrating experience. It suggests a technical issue with your email's structure, rather than its content or recipient engagement.
Microsoft Outlook, can cause issues when converting Rich Text to HTML.
