We generally leave any header that you add to your message intact in the outbound message. We do, however, remove any Contactlab-specific header that matches the
X-Clab-(.*) expression. As a result, you should ensure that you never create custom headers for your own use with a key that matches this expression.
The following headers are always added by Contactlab:
Emails sent to gmail.com recipients have two additional headers:
This is needed to enable the Gmail aggregated feedback loop.