2nd Exchange Srvr queuing incoming mail

I trying to find out if this is possible, and if so how to go about doing it. I’m running BackOffice 4.0 SP6a with Exchange 5.5 SP3 on the PDC. I have installed Exchange on a BDC joining the PDC.

What I want to happen is that when the PDC ever goes down that the incoming mail then get routed over to the BDC and held there until the PDC comes back up.

Let me know if you need more information. Thanks for your help.

Ken