How to add custom headers on sent emails?

I don’t see where its possible to add custom headers to outgoing emails? Specifically, “List-Unsubscribe”. If its not possible, is this on the roadmap?

Reading the eM Client threads, it appears there is no option to add / or change headers on outgoing messages, only modify / edit the source html code. So all depends if enough other peeps would also want this feature.

Good day, this is also needed for our email in combination with Sophos XGS Firewall, we want to add an spx record so emails are very easily converted to secure pdf’s with password protection. Any idea if this can be established?