[Exim] Where does mail go to die^H^H^H^H bounce?

I'm a little confused: I understand that mail that's undeliverable will sit
frozen in the queue, attempting to deliver... but isn't there some timeout or
something that I can tweak, past which the e-mail will be bounced back to
sender? How does that work?

Thanks,

Ken D'Ambrosio
Sr. SysAdmin,
Xanoptix, Inc.