Why are my Gmail emails bouncing with a 'missing Message-ID' error?
Michael Ko
Co-founder & CEO, Suped
Published 21 Apr 2025
Updated 28 Aug 2025
6 min read
Email deliverability can often feel like a moving target, especially when mailbox providers like Google Gmail introduce stricter compliance requirements. Recently, many senders have encountered a puzzling bounce message from Gmail: "Messages missing a valid Message-ID header are not accepted." This can be particularly frustrating when you believe your Message-IDs are correctly formed.
This specific error indicates that the email’s Message-ID, a crucial header that ensures email uniqueness, is either absent or syntactically invalid according to RFC 5322 specifications. While this has always been a standard, Gmail (and other major providers like Outlook and Yahoo) is increasingly enforcing these rules to combat spam and ensure legitimate mail delivery. We'll explore why this is happening and what you can do to resolve it.
Understanding the message-id bounce
The Message-ID is a header field in an email that uniquely identifies a specific message. It’s critical for email tracking, preventing duplicate deliveries, and facilitating responses in a conversation thread. According to RFC 5322, the Message-ID must be globally unique and follow a specific format, typically resembling an email address: a local part followed by an "@" symbol and a domain part. An example is `<unique-string@yourdomain.com>`.
Even if your Message-ID generally looks correct, subtle syntax errors can trigger Gmail’s strict checks. One common mistake we've observed is the inclusion of invalid characters, such as colons (:) within the local part of the Message-ID, which are not permitted by RFC 5322. If your client or sending system generates Message-IDs like `<platformname.EMAIL:some-uuid@domain.com>`, the colon breaks compliance.
While RFC 5322 recommends including a domain identifier on the right-hand side of the "@" to ensure uniqueness, there’s no explicit requirement for the Message-ID’s domain part to align with the sender’s domain. The primary concern for mailbox providers is that the Message-ID is unique and syntactically valid. Violations primarily result in delivery failures, leading to hard bounces.
Why Gmail is increasing enforcement
Gmail and other major mailbox providers are continuously enhancing their spam filtering capabilities and tightening their email sending guidelines. This includes stricter enforcement of long-standing RFC specifications. Emails that previously might have slipped through with minor compliance issues are now being rejected outright, especially if a sender has a high volume of such non-compliant messages.
This increased rigor is part of a broader industry trend to improve email security and reliability. By rejecting malformed emails, providers aim to reduce spam, thwart phishing attempts, and ensure legitimate communications reach their intended recipients. It’s a move towards a cleaner email ecosystem, but it places a greater burden on senders to ensure their email infrastructure is perfectly configured.
The fact that some emails in a batch are accepted while others bounce suggests that Gmail may be gradually rolling out or testing these stricter compliance checks. This means that even if your emails currently deliver without issue, a future increase in enforcement could lead to widespread bounce problems. It’s a clear signal to proactively address any known RFC violations now.
This also applies to other headers beyond just Message-ID. For instance, a missing or invalid From: header can lead to similar rejections because it prevents proper DMARC authentication. Mailbox providers are increasingly focusing on a holistic view of email authenticity and compliance.
Steps to resolve missing message-id errors
The primary step to resolve a "missing Message-ID" bounce is to audit your email generation process. If you’re using an Email Service Provider (ESP) or a marketing automation platform, work with them to ensure their system generates Message-IDs that are strictly RFC 5322 compliant. This often involves checking the internal libraries or code responsible for creating these headers.
Specifically, look out for non-standard characters in the local part of the Message-ID. The standard allows for alphanumeric characters, periods, hyphens, and underscores. Any other characters, like colons, can cause the Message-ID to be considered invalid. Make sure the ID is structured as a unique string followed by an "@" and a domain.
If you control your email sending infrastructure, for instance, using a Mail Transfer Agent (MTA) like Postfix, examine its configuration and any custom scripts that modify email headers. Ensure that the Message-ID is being correctly added and formatted before emails are sent. Logging the full Message-ID that goes out is crucial for debugging.
Incorrect Message-ID
Syntax Issue: Contains characters (e.g., :) not allowed by RFC 5322 in the local part.
Uniqueness: May not be globally unique, though less common for this specific error.
Correct Message-ID
Syntax Correct: Adheres strictly to RFC 5322, using only allowed characters.
Format: Looks like <local-part@domain>, with the domain typically being the sender's domain or a subdomain.
I recommend using a tool like an email deliverability tester that provides full header analysis. Sending a test email and examining its raw headers will quickly reveal if the Message-ID is indeed malformed. You can also refer to external resources for guidance on Message-ID syntax to validate your implementation.
Broader deliverability impacts
While fixing the Message-ID issue is crucial, it’s also important to remember that email deliverability is a multi-faceted challenge. A valid Message-ID is just one piece of the puzzle. Other factors like sender reputation, email authentication (SPF, DKIM, DMARC), content quality, and recipient engagement all play significant roles.
If you're experiencing various Gmail bounce messages (e.g., unsolicited mail or DKIM/SPF failures), it often indicates a broader underlying issue with your email program. Monitoring your blocklist status and proactively checking your domain’s health are essential for consistent inbox placement.
Views from the trenches
Best practices
Ensure your Message-ID generation strictly adheres to RFC 5322. Use a UUID or timestamp combined with your domain.
Regularly test your email headers using a reliable email deliverability tool to catch compliance issues early.
Monitor your domain’s reputation metrics, including spam rate and complaint rate, through Google Postmaster Tools.
Implement strong email authentication (SPF, DKIM, DMARC) and ensure alignment for all sending domains.
Proactively review your email sending practices as mailbox providers continually update their requirements.
Common pitfalls
Assuming Message-IDs are fine because they were accepted previously; enforcement changes frequently.
Ignoring subtle syntax errors (e.g., colons in the local part of the Message-ID).
Not having a mechanism to inspect outbound email headers for compliance.
Focusing only on Message-ID while overlooking other critical factors like sender reputation and authentication.
Failing to adapt to evolving sender requirements from major mailbox providers like Google and Yahoo.
Expert tips
If using an ESP, confirm their Message-ID generation aligns with current RFC standards. They should handle this for you.
For custom sending systems, implement robust validation checks for Message-ID format before sending.
A sporadic bounce pattern suggests a gradual rollout of new enforcement rules. Act quickly to avoid wider impact.
Always maintain logs of bounced messages, including full headers, for detailed troubleshooting.
Don't solely rely on bounce messages. Proactive monitoring of email deliverability metrics is key.
Expert view
Expert from Email Geeks says that Google's code for detecting Message-IDs might be buggy or the Message-ID generator is occasionally creating invalid Message-IDs, or even that the rejection message itself might be misleading.
2024-08-01 - Email Geeks
Expert view
Expert from Email Geeks says that including a colon (:) in a Message-ID is not syntactically valid according to RFC standards.
2024-08-02 - Email Geeks
Maintaining email health
A "missing Message-ID" bounce from Gmail is a direct signal that your emails aren't fully compliant with industry standards. While it might seem like a minor technicality, mailbox providers are leveraging these standards to filter out potentially malicious or poorly constructed email. Addressing this issue promptly is crucial for maintaining good deliverability.
By ensuring your Message-IDs are correctly formatted and globally unique, you not only resolve the immediate bounce problem but also contribute to a stronger overall email reputation. This proactive approach will help you navigate the evolving landscape of email deliverability and ensure your messages consistently reach the inbox, avoiding the spam folder or a hard blacklist.
