[exim] Redirection

Hi,
Perhaps an obvious question, but could anyone tell me at the technical level
how redirection works with exim ?

Reason I ask is this. We run a mail server for a client, and they are about
to run a promotion for something which involve vasts quantities of mail that
will go to another company. Basically I don't want to receive those mails
into our mail server, 'cause it'll probably bring it to a halt, so I just
want exim to redirect the mails immediately, and not receive them, and then
redirect them.

That make sense to anyone ? Apologies if I've not explained that to well. It
might be that I've just not understood how redirection works at the basic
level.

Thanks

Mike