When you change the IP of an "inner" mail server. You need to perform the following:

 1. Change /etc/postfix/transport (to send the mail to the right server)
 1. Change /etc/postfix/mynetworks (to be able to forward mail from your new IP)
 1. postmap transport //Compiles the transport file into transport.db - you don't need to do this for mynetworks

Concepts:
 * Change email server IP 
 * Smart Host for dynamic IP
 * smarthost