If you have to install postfix in a multi-IP environment, say, if you need it to listen to two or more IP (for example a real IP and an alias on the same NIC), there is a cute directive in main.cf that let you decide which IP should postfix (well, it’s parts like smtp, virtual/maildrop etc) use when contacting an external server.
inet_interfaces = 192.168.1.200, 192.168.1.201, localhost
smtp_bind_address = 192.168.1.201
with inet_interfaces your postfix will listen to these 3 IPs, and with smtp_bind_address you will tell postfix to specifically use 192.168.1.201 when contacting an external address.
The eternal fight between admins and computers 