Mailips is mainly auto-generated file and is used for outgoing SMTP interface. However, you can also manually update and manage the mailips file. Full path to the mailips is: "/etc/mailips"
The syntax is:
The above line will send outgoing mail from the default (eth0) IP address. Entry begins with an asterisk (*) will sets the default sending IP (from the eth0) address for domains without their own specific entry.
Each line defines a personal outgoing server for the domain. Doing it this way, each domain can have its own outgoing IP and that way, you will be able to setup RDNS (Reverse DNS) for each IP for that domain name.
Please note: IP address must be public IP addresses that are accessible via the Internet.