Each Postfix option in the Webmin module is documented. Just click on the option's name and a new window will appear, explaining the relevant option.
Postfix's configuration begins with the first icon: General options. The Most useful general options part is relevant and interesting. The other part will be described in the the section called “Advanced Configuration” section of this chapter. The first option concerns outgoing mail. You should specify the mail domain. Leave it to domainname if the computer's domain name has the same value as your mail domain name. Through the second option, Postfix knows which domains and host names to manage. For example, your computer name is gateway.example.com and your mail domain is example.com. The outgoing mail domain should be example.com and the incoming domains would be example.com, localhost.example.com, localhost.localdomain, gateway.example.com: these are Webmin default values. The last important option is the Postmaster address. In that field, you must specify the administrator's e-mail address, whom will receive mail reports in case of problems.
The options contained in the second part, Other general options, must be configured. The Internet hostname of this mail system must be set up with the correct hostname. A wrong entry could lead your e-mails to be rejected from other mail servers. The Local Internet domain name option must be specified. Finally, by specifying the correct Local networks, you could avoid becoming the victim of illegal spammers. This option tells Postfix from which IP address it should accept e-mails. Leave all the notice options to the postmaster. He will receive all error messages from Postfix.
The Mail aliases section configures the mail redirection to valid existing mailboxes. For example, you can configure it in order for all mail sent to the postmaster to be redirected to root's mailbox. You can also configure e-mail aliases for your users. The first part specifies where Postfix should look for in the database file.
The next step concerns General resource control. Two options are interesting: Max size of a message and Max size of bounced message. The first one configures the maximum size of e-mails managed by Postfix. It restricts the file size of attachments users of your network could try to send. The second option avoids to fill your hard drive when an e-mail cannot be delivered locally. It bounces in Postfix's queue, waiting to be delivered.
In the SMTP server options section, configure Restrict mail relaying with these values: $mydestination,$mynetworks. It will prevent Postfix from relaying e-mails from any client.