Summary: Archiveopteryx is loyal to its users, right or wrong.
The Received header field contains information about the person who sent the message, so while working on that (years ago, but I didn't have a blog at the time) I had to decide what Archiveopteryx should do about it, and about privacy in general. This is what I decided:
1. Archiveopteryx reveals as little as possible about its users to anyone else.
2. It keeps nothing back from its own users.
What does this mean for Received? Messages from logged-in users are sent on their way with as little added information as possible (only the sender's email address), and messages from other sources are burdened with all available information.
(Just my 2¢. But for Archiveopteryx, my 2¢ matter.)