Following this Support request, i propose to add a setting into Mail module.
The setting is about cut or hide some data from the original sender into the header.
Name of the setting: full sender data into header.
Default option: current setting, no modification to the headers.
External only: if the message is not delivered into a local mailbox, hostname and sender ip address are removed or obfuscated into headers.
All: any message won’t contain hostname or ip address of the sender.