The mynetworks parameter lists all networks that this machine somehow trusts. This information can be used by the anti-UCE features to recognize trusted SMTP clients that are allowed to relay mail through Postfix.
You can specify the list of trusted networks in the main.cf file, or you can let Postfix deduce the list for you. The default is to let Postfix do the work for you.
mynetworks_style = subnet
The meaning of the styles is as follows:
Trust SMTP clients in the class A/B/C networks that Postfix is connected to. Don't do this with a dialup site - it would cause Postfix to "trust" your entire provider's network. Instead, specify an explicit mynetworks list by hand, as described below.
Trust SMTP clients in the IP subnetworks that Postfix is connected to.
Trust only the local machine.
Alternatively, you can specify the mynetworks list by hand, in which case Postfix ignores the mynetworks_style setting. To specify the list of trusted networks by hand, specify network blocks in CIDR (network/mask) notation, for example:
mynetworks = 188.8.131.52/28, 127.0.0.0/8
You can also specify the absolute pathname of a pattern file instead of listing the patterns in the main.cf file.
My own network addresses
The inet_interfaces parameter specifies all network interface addresses that the Postfix system should listen on; mail addressed to user@[network address] will be delivered locally, as if it is addressed to a domain listed in $mydestination.
The default is to listen on all active interfaces. If you run mailers on virtual interfaces, you will have to specify what interfaces to listen on.
You even have to specify explicit machine interfaces for the non-virtual mailer that receives mail for the machine itself: the non-virtual mailer should never listen on the virtual interfaces or you would have a mailer loop.
mail.err is for any critical Postfix errors. If you are having troubles with Postfix not starting, crashing, or otherwise not working in a critical way, this is where you will find messages about it. mail.info contains information about Postfix, it's processes, and tasks. mail.warn contains non-critical warning errors with respect to Postfix. For example, if you do not have a FDN, you may see complaints in this file.