Starting April 24, 2023, Gmail will begin rejecting messages that contain more than one RFC-mandated single instance header in order to better protect you from spam and abuse.
What does this mean for your institution?
If vendors and other institutions your school or university communicates frequently with are currently sending messages with multiple headers, the messages will start being rejected with the error message: “This message is not RFC 5322 compliant,” starting April 24, 2023.
For context, email headers are a set of lines that precede the body of an email message. They contain information about the sender, recipient, subject, the message's route to the recipient's inbox, and how to interpret the body (text, html, image). They also provide information that can be used to verify the authenticity of a message. Therefore, rejecting messages that contain multiple headers protects you from malicious duplicate header exploitation.
The Internet Official Protocol Standards: Internet Message Formats [Request For Comments (RFC) 5322] states that a message must have at most one instance of each of the following headers:
- To
- Cc
- Subject
- Date
- From
- Sender
- Reply-To
- Bcc
- Message-ID
- In-Reply-To
- References
Note: Please see RFC Editor’s Internet Message Format RFC 5322 document for additional information on the headers that are standards-compliant.
What do you need to do?
We recommend distributing this message to vendors and other institutions your school or university communicates frequently with to avoid being impacted by this effort. For more information on support please see Prevent mail to Gmail users from being blocked or sent to spam.