Now a mail file has an option of "Always show details in received Mail messages." $mHeaderDetails=1 is set in your notes.ini when you enable it and open a message in the mail file.
However, $mHeaderDetails=1 is deleted when you open a message of another mail file whose "Always show details in received Mail messages" is unchecked.
The header is hidden again when you open a message in your mail file after $mHeaderDetails=1 is deleted, though $mHeaderDetails=1 is set again then.
This causes inconsistent behavior for users.
I request Notes to have an ability to enable "Always show details in received Mail messages" in each Notes client so that you always see details of headers in all mail files.