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
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)
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.
