Enabling this option will preserve any custom headers that are a part of your original mail message. The SMTP relay previously would strip off custom headers before teh email was delivered to the recipient. With this feature, if your email message includes custom headers or headers added on by an email client like:
X-MSMail-Priority
User-Agent
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:
X-CustomEmailID
those will also be preserved.
We will be enabling an option in a future launch 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.
