What happened to me was that an autoresponse "bounced". The bounce
message still had the X-Loop, but it was not part of the header, since the
other guy adds a header and quotes the "bounced" message. I can also
check the body for my X-Loop to fix that. (Or does someone see a
potential problem with that?)
Did the bounce not match the FROM_DAEMON?
Yes, it did. I sent an autoresponse, adding an X-Loop: <my ID>
The destination host bounced 'no such user'
Early in my .procmailrc I find and discard messages with that exact X-Loop
header. A bounce message does not have that in the header section, it has
it after "Original Message Follows"
So the message was subsequently shunted into my folder for bounces. I
don't auto-discard bounce messages--one might tell me that a friend has
re-located. :-) But as long as I don't auto-respond to such a bounce
message, I have at least accomplished what I intended X-Loop to do.