The “Referer” header is a HTTP header that is added by the web browser whenever a request is made. A Zimbra user who receives an email with links or images in the Zimbra web interface may unknowingly share information of the Zimbra server when clicking the link or viewing inline-images.
For example when an email is forwarded from one organization to another organization the final recipient may click the link, and then the Zimbra server url is passed to the server where the link points to.
The Referrer-Policy HTTP header controls how much referrer information (sent with the Referer header) should be included with requests. You can disable the Referer header in Zimbra as follows:
sudo su - zimbra zmprov mcf +zimbraResponseHeader "Referrer-Policy: no-referrer" zmcontrol restart