Enabling this option will preserve any custom headers that are a part of your email message. Prior to this feature, custom headers would be stripped off by the SMTP Relay before the email was delivered to the recipient. Now, if your email message includes custom headers or headers added on by an email client like:
they will be included in the final email sent to the recipient. Additionally, if you have custom headers that help identify an email message in your internal system, such as:
those will also be preserved.
In the future, we'll be enabling an option such that you can set a particular custom header's value to show in Reports, allowing you to tie each individual sent email, open, and click back to your internal system's own identification mechanism.