[mew-int 3030] New mails notification

Hi,
I'm a new mew user, I've searched all around but cannot find out the solution.
I have:

    (setq mew-use-biff t)
    (setq mew-use-biff-bell t)
    (setq mew-biff-interval 5)

in my .mew.el. 
The question is: "How can I call a function or an external program when new mails arrive?"

Thanks for reading.