In case you want cmail to use only a specific IP address for outgoing connections,
network namespaces are a rather simple way to do this.

You probably want this reboot safe.
I recommend setting this up via your systems network config.
There is an example config for Debian below.

1) Setting up the namespace

The manual steps:

Add a new bridge.
# ip link add br0 type bridge

Add your public interface to the bridge.
!!! WARNING! Doing this will probably break your network connection! !!!
Make sure you still have access to the system via a different interface.
# ip link set dev eth0 master br0

Assign an IP to the public interface. (e.g. via DHCP)
# dhclient br0

Setup a network namespace for cmail.
# ip netns add cmail-ns

Add a pair of virtual ethernet interfaces.
# ip link add veth0 type veth peer name veth1

Add one end to the bridge.
# ip link set dev veth0 master br0

Add the other end to the network namespace.
# ip link set dev veth1 netns cmail-ns

Bring both interfaces up.
# ip link set dev veth0 up
# ip netns exec cmail-ns ip link set dev veth1 up

Add an IP address to the interface in the new namespace.
# ip netns exec cmail-ns ip address add 198.51.100.15/24 dev veth1

Add a route to the namespace.
# ip netns exec cmail-ns ip route add default via 198.51.100.1 dev veth1


### Example interfaces(5) config for Debian.

auto br0
iface br0 inet dhcp
	bridge_ports eth0
	bridge_stp off
	bridge_waitport 0
	bridge_fd 0

auto veth0
iface veth0 inet manual
	pre-up ip netns add cmail-ns
	pre-up ip link add veth0 type veth peer name veth1
	pre-up ip link set dev veth0 master br0
	pre-up ip link set dev veth1 netns cmail-ns
	up ip link set dev veth0 up
	up ip netns exec cmail-ns ip link set dev veth1 up
	post-up ip netns exec cmail-ns ip address add 198.51.100.15/24 dev veth1
	post-up ip netns exec cmail-ns ip route add default via 198.51.100.1 dev veth1

	pre-down ip netns exec cmail-ns ip link set dev veth1 down
	down ip link set dev veth0 down
	post-down ip link del veth0
	post-down ip netns del cmail-ns


2) Run cmail in the namespace

The last step is to run the cmail instances in the new namespace.

# ip netns exec cmail-ns /usr/sbin/cmail-msa /etc/cmail/msa.conf
# ip netns exec cmail-ns /usr/sbin/cmail-mta /etc/cmail/mta.conf
# ip netns exec cmail-ns /usr/sbin/cmail-popd /etc/cmail/popd.conf