Qmail server set to forward to another host on the local ethernet.

[Rawlinsons Group \(Brisbane\)]

I haven't done this yet so I can't provide real internet names.

I will have the following:

Computer A (On ISP Primary)
 | - Linked via 100Mbps ethernet
Computer B (On ISP Secondary)

I'd like to setup A as the first MX record for host blah.com and B as the
secondary MX record for blah.com both on different ISPs and connected via
100Mbps ethernet.

If A is down and B starts receiving email for A, how do I forward all the
emails B receives to A over the ethernet and not keep copies of those emails
on server B?

Thankyou very much.