Since I’m always forgetting, the following is necessary on the host box if you’re running virtual machines w/ networking configured in NAT mode:
# iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
This also requires:
# sysctl -w net.ipv4.ip_forward=1
The entry below simply port forwards 2222 on the host to the virtual machine:
# iptables -t nat -A PREROUTING -p tcp --dport 2222 -j DNAT --to 192.168.237.2:22
In this example, we’ll say the additional IP we want to route mail out through is 1.2.3.4.
Now, there are two ways to do this. The first is to use iptables:
# iptables -t nat -A POSTROUTING -p tcp --dport 25 -j SNAT --to-source 1.2.3.4
The second, assuming you’re using sendmail, can be done by adding the following to /etc/mail/sendmail.mc:
define(`confCLIENT_OPTIONS',`Address=1.2.3.4')dnl
Proceed to rebuild the /etc/sendmail.cf file and restart sendmail.