DON'T DELETE THIS MESSAGE -- FOLDER INTERNAL DATA

[Mail System Internal Data <MAILER-DAEMON xxxxxx com>]

This text is part of the internal format of your mail folder, and is not
a real message.  It is created automatically by the mail system software.
If deleted, important folder data will be lost, and it will be re-created
with the data reset to initial values.

---snip--snip---

Pine didn't like the mbox until I put that at the front (complete with
trailing blank line), followed by the mbox that evolution had built.