SSH in to your server console and let’s look at your setup:

# /sbin/ip route
1.2.3.4/1 dev eth0 proto kernel scope link src 1.1.1.1
 1.2.3.4/16 dev eth0 scope link
 default via 3.3.3.3 dev eth0

To change the default qmail IP from 1.1.1.1 to 2.2.2.2:

# /sbin/ip route change default via 3.3.3.3 dev eth0 src 2.2.2.2

That’s all there is to it. All qmail outgoing mails now leave from IP 2.2.2.2 instead of 1.1.1.1