Understanding email headers: x-beenthere and x field

Question

I'd like to understand more about the email headers.

After looking at one of the emails, I found the field had this entry "x-BeenThere" in the header. What does this mean?

Also I noticed some "x" fields seem like they're custom set fields, is this the case with some emails?

Answer 1

The X-BeenThere header is used by mailing list software to detect loops. Each time a message passes through a listserve, it adds its address to the header. If an incoming message already contains its address in the header, it means the message is looping (one of the recipients is forwarding their email to the list) and it's dropped.