Troubleshoot RFC 5322 (duplicate header) bounce messages

Take steps to prevent sending or receiving messages with duplicate headers

Email headers are part of an email message that has information, or metadata, about the message. For example, headers contain information about the message sender, recipient, and time sent. Email systems use headers to authenticate messages and senders.

Spammers and other malicious senders add duplicate headers to messages to impersonate legitimate senders. To protect people in your organization, Gmail blocks messages with more than one header of the same type, or duplicate headers.

When Gmail blocks a message with duplicate headers, Gmail sends this error, or bounce, message to the sender: This message is not RFC 5322 compliant. RFC 5322 is an Internet standard that defines email message format, including message headers.

Sometimes legitimate, non-malicious messages contain duplicate headers. This typically happens with devices or systems that send automated email messages.

This article describes the duplicate headers that Gmail checks for, and has tips on how to Resolve issues with duplicate headers.

More about duplicate headers

Duplicate headers Gmail checks for

When checking for duplicate headers, Gmail checks these headers:

To:, Cc:, Subject:, Date:, From:, Sender:, Reply-To:, Bcc:, Message-ID:, In-Reply-To:, and References:

By default, when Gmail identifies a message with duplicate headers, it blocks the message from being delivered to the recipients, and sends this error, or bounce, email message to the sender: This message is not RFC 5322 compliant.

Duplicate header example (picture)

In the examples below, the message with correctly formatted headers has one To header in the message. The To: header has 3 recipient email addresses, separated by commas. In the duplicate header example, the To: header appears 3 times, once for each recipient. Gmail will block this message from being delivered.
Duplicate headers.

Resolve issues with duplicate headers

Outgoing messages: If you identify outgoing messages with duplicate headers, update the email format used by those senders so outgoing messages follow RFC 5322, and are delivered as expected.

Incoming messages: If you people in your organization get incoming messages with duplicate headers, let senders know about the issue so they can resolve it. Important: Starting in April 2024, Gmail rejects incoming messages that have duplicate headers, so incoming messages with duplicate headers might not be delivered to recipients as expected. Learn more

I’m not a Google Workspace customer, but I’m getting RFC 5322 bounce messages: We recommend you use any tools available to you to identify senders of messages with duplicate headers. Let these senders know about this issue so they can resolve it.

Related topics

Troubleshoot Gmail not getting contact form messages

RFC 5322 Internet standard

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