Force postfix to send on specific IP

I have a server with multiple IPs, I can't work out how to force postfix to send via one of our virtual IPs rather than the main IP. (For example main IP is 1.2.3.4, virtual IP is 4.3.2.1)

I've tried this in main.cf:
inet_interfaces = 4.3.2.1

I've tried this in master.cf

Code:

4.3.2.1:smtp      inet  n       -       n       -       -       smtpd

Using Postfix 2.3.3 on RHEL5

Can anyone tell me how to force postfix to send on 4.3.2.1 ?

 

you may want to use smtp_bind_address. man 5 postconf and look for it

 

I've tried that and it looks like it should bind to inet_interfaces. Neither of which seem to work.

 

If it does not work then use NAT to rewrite the source address of the smtp packets before they leave your server.