The documentation states:

"Several variations of the mbox format exist to address perceived shortcomings in the original. In the interest of compatibility, mbox implements the original format, which is sometimes referred to as mboxo."

http://docs.python.org/dev/library/mailbox.html#mailbox.mbox

But this format is fundamentally broken, corrupting lines that start with "From ", and I can't find any justification for using it in Python.  In fact, all four links included in that section argue against this format.

If only one mbox format is used, it should be mboxrd.  Otherwise, include support for all the variants, with mboxrd as the default.