Qmail cluster

Hi all

We have a high email load (approx 4 million emails each day). Right now we
solve this by having multiple independent qmail servers. My idea is to set
up a high abalability/load balacing cluster that has a virtual ip address
and then change our MX record to point to this virtual ip.

My problem is now, that in this cluster each server will assing its own
physical ip the out going emails. This will require me changing each of
our firewalls to accept smtp traffic from and entire range of ip's instead
of just one.

What I am looking for is a way for each of the real servers in the cluster
to send mail using the same virtual ip as the cluster use.

Can this be done ?