Privacy Forest

It turns out Mail's "Mail Privacy Protection" warns you it does not work, this can be because you have Network's "Limit IP Address Tracking" enabled (it also doesn't work with proxy's and VPNs). The latter works only in Mail and Safari, so if you use another main browser you could simply disable the "Limit IP Address Tracking" option and benefit from Mail privacy. How both these options work with iCloud Private Relay on Mac I'm not sure.