According to the de-facto MBOX standard [1] and the MMDF
description [2] mtime and atime are used to detect wether
a mailbox has new mail:

   If the mtime on a nonempty mbox file is greater than the
   atime, the file has new mail.

For [1] this is documented under "UNSPECIFIED DETAILS", though.
The attached patch enables MUAs like mutt(1) to show the
new-mail-has-arrived status in the overview, too.
Note i've randomly chosen 3 seconds; it also could have been 5.

[1] http://qmail.org/man/man5/mbox.html
[2] http://linux.die.net/man/5/mmdf