If we want to change the qmail interface IP (for example from 192.168.0.1 to 192.168.0.2) below steps would be helpful for you :-
Check the current interface IP and gateway IP by typing the following from the CLI.
#/sbin/ip route
We gets an output like -
=====
x.x.x.x/27 dev eth0 scope link metric 1002
x.x.x.x/16 dev eth0 proto kernel scope link src 192.168.0.1
default via 192.168.0.20 dev eth0 proto static
=====
Here the interface ip is 192.168.0.1 and gateway IP is 192.168.0.20
Use the following command to change the default qmail IP from 192.168.0.1 to 192.168.0.2
/sbin/ip route change default via 192.168.0.20 dev eth0 src 192.168.0.2
Send a test email and check the header to see if the mail coming from new IP.
