Richard Heyes has posted about a mail() replacement he's come up with that adds the additional functionality of backing up all of the data to a certain directory of your choosing.

A simple drop in replacement function for PHPs mail() function called mailb() which backs up all the data to a specified directory. [...] I've also added a simple header file to Apache and the download directory so it looks nicer.

You can download a copy of his library here as a .phps file (a quick and easy 40ish line script).