What do Apple X-Headers mean in email filtering?

Marketer from Email Geeks assumes that CLX is based on cloudmark or proofpoint.

Email marketer from EmailGeek Forum mentions that X-Apple headers, like `X-ICL-Score`, give insight into how Apple's system evaluates email for spam. A higher score means a greater chance the email is spam.

Email marketer from Cybersecurity Firm explains that X-headers may indicate phishing attempts by revealing suspicious domains or unusual sending patterns, acting as red flags for security analysts investigating potential email threats.

Marketer from Email Geeks mentions that CLX is Proofpoint's secret sauce, IIRC.

Email marketer from Stack Overflow explains X-headers allow developers and email admins to implement custom email filtering rules based on specific criteria, such as tagging or routing certain types of messages. This is often leveraged in custom email solutions.

Email marketer from Email Provider Forum states that X-headers can dictate how emails are routed internally within an email provider's infrastructure, enabling customized delivery paths based on header data. This enhances email management efficiency.

Email marketer from Email Marketing Agency describes X-headers are used to track marketing campaign performance by embedding unique identifiers that link back to specific campaigns, enabling detailed analytics and performance measurement.

Email marketer from Email Deliverability Service explains X-headers related to deliverability show issues that might affect where the email ends up. This can include server info, bounces, or feedback loops.

Email marketer from Reddit describes that Apple uses X-headers like X-Apple-Action to indicate the outcome of spam filtering, whether the email was moved to junk or kept in the inbox, giving users insight into Apple's assessment.

Email marketer from Email Security Blog explains that X-headers are used to show details of email security scans, they can indicate if a message has been scanned for malware and what the results were. This gives some insight into how secure the email is.

Expert from Spam Resource mentions that X-headers are used to store data and can be used to identify spam. Different X-headers can indicate a sender's reputation, the results of spam checks, or the reasons why a message was classified as spam.

Expert from Email Geeks shares that the Apple X-Headers are similar to MSFT headers and indicate the filtering decision on the message, providing examples for both INBOX and Junk folders.

Expert from Email Geeks says that Icloud used to have a bunch of X-CLX-Whatever headers for their spam filtering metadata, so it’s something that’s been around a while.

Expert from Word to the Wise explains that different email providers and filtering systems use X-Headers to add proprietary information about how they have filtered the email. These are custom implementations and can include details about spam scores, classification rules and any other kind of meta-data.

Documentation from Proofpoint Communities explains that some X-headers contain CLX classifications, revealing how Proofpoint's technology categorizes emails based on threat levels, content, and source reputation.

Documentation from SpamAssassin Wiki says X-Spam-Flag is a common header used to signify whether an email has been marked as spam by the SpamAssassin filtering system.

Documentation from ietf.org explains X-headers are non-standard header fields that can be used to add custom information to email messages, where X- means they are non-standard additions to the email headers for filtering and categorisation.

Documentation from Apple Support explains that `X-Apple-MoveToFolder` indicates the destination folder for an email (e.g., INBOX, Junk), and `X-Apple-Action` specifies the action taken on the email (e.g., MOVE_TO_FOLDER).