Email headers are part of an email message that provides information, or metadata, about the message. For example, email headers contain information about the sender, recipient, and the time the message was sent. Email systems, including Gmail, use the information in message headers to authenticate email messages and senders.
To protect people in your organization from malicious or spam emails, Gmail blocks messages that contain more than one header of the same type, or duplicate headers. Spammers and other malicious senders use duplicate headers in messages to impersonate legitimate senders. Gmail blocks these messages to protect email users from spam and phishing messages.
When Gmail blocks a message with duplicate headers, Gmail sends this error, or bounce, email message to the sender: This message is not RFC 5322 compliant. RFC 5322 is an Internet standard that defines the correct format for email messages, including message body, headers, and attachments.
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 includes recommended action for messages with duplicate headers.
More about duplicate headers
When checking for duplicates, Gmail checks these headers in messages:
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
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 identify incoming messages with duplicate headers, let senders know about the issue so they can resolve it.
I’m not a Google Workspace customer, but I’m getting RFC 5322 bounce messages: We recommend you use the admin tools that are available to you to identify senders of messages with duplicate headers. Let message senders know about the issue so they can resolve it.