Linux Postfix binary packages typically store configuration files in /etc/postfix. The main configuration file in this directory is main.cf , which controls the overall Postfix configuration. This file consists of comments, which are denoted by lines beginning with hash marks (#) and option lines of the form:

                  variable = value

The variable is typically a descriptive name, such as relayhost to set the hostname of another SMTP server that’s to act as a mail relay. The value can be a hostname, IP address, filename, or other string. Sometimes a value can have multiple components, separated by commas. A value can also refer to an earlier variable by name: precede the earlier variable name by a dollar sign ($), as in myorigin = $mydomain to set the myorigin variable to be identical to mydomain.

The default main.cf file is extremely well commented, so you can learn a great deal about the configuration and how you can change it by reading the comments. Further information, including information on obscure options not mentioned in the default comments, can be found in the online Postfix documentation.

In addition to the main.cf file, Postfix relies on several other configuration files. Most of these are binary database file with filenames ending in .db. These files are similar in purpose to sendmail’s database files; they control username aliases, relay host mapping, and so on. Like the sendmail files, the Postfix database files are generated from plain-text files that typically take the same name as the database file but without the .db extension. Some of these files are described in upcoming sections.

The Postfix address options begin with setting the server’s name. As with sendmail, Postfix uses gethostbyname( ) to determine the computer’s hostname and sets the hostname it reports to other systems appropriately. You can override this feature by setting myhostname:

myhostname = smtp.pangaea.edu

Two related options are mydomain and myorigin. The first of these sets the server’s Internet domain; it defaults to the value of $myhostname minus its first component, as in pangaea.edu if $myhostname is smtp.pangaea.edu. The myorigin variable sets the hostname that Postfix appends to email addresses that don’t specify a hostname. The default value is $mydomain, but you can change this to $myhostname or any other value, as appropriate.

If you want to force outgoing mail to have a particular return hostname, you can use the masquerade_domains option. You pass a domain name to this option, and hostnames within that domain are stripped down to the domain portion. For instance, if you set this option to pangaea.edu, and a user sends mail that has a return address of linnaeus@gingko.pangaea.edu, Postfix changes the outgoing address to linnaeus@pangaea.edu. This can be a handy option for coping with clients that insist on adding their own hostnames to outgoing mail. Mail with return addresses outside of the http://pangaea.edu domain are unaffected by this line, though. The masquerade_classes option affects the precise parts of the mail that are affected. You can set this to one or more of envelope_sender (the sender in the mail envelope), header_sender (the sender in the mail header), and header_recipient (the recipient in the mail header, typically used to strip hostnames from incoming mail). Typically, one or both of the first two options is used.

Still more complete address rewriting can be accomplished with the help of a file called sender_canonical. You specify the use of this file with the sender_canonical_maps option in main.cf:

sender_canonical_maps = hash:/etc/postfix/sender_canonical

You then edit the sender_canonical file so that each line holds an original email address or address fragment followed by the address or matching fragment you want substituted:

FETCHMAIL-DAEMON@localhost postmaster@pangaea.edu
@mandragora.example.com @pangaea.edu

These lines tell Postfix to replace FETCHMAIL-DAEMON@localhost with postmaster@pangaea.edu and to change any address at http://mandragora.example.com with the matching address at http://pangaea.edu. Once you’ve edited this file, type postmap sender_canonical. This command creates a sender_canonical.db file from the text-mode sender_canonical file.

When receiving mail, Postfix uses the mydestination variable to determine what addresses it’s to treat as local. Mail addressed to any user at any of the $mydestination addresses is passed to local users; mail addressed to other addresses is relayed to that address, assuming relaying is authorized. You can set multiple hostnames for mydestination by separating them with commas, as in:

mydestination = $myhostname, localhost.$mydomain, mail.pangaea.edu

Most default Postfix configurations relay mail from the local network and deliver mail directly to the destination server without using an outgoing relay. Thus, if you want to fine-tune your relay configuration or use an outgoing mail relay system, you must adjust your Postfix configuration. You may also want to make changes if you want Postfix to deliver incoming mail to other servers, such as to Microsoft Exchange servers, using Postfix as a spam filter, mail sorter, or in some role other than the final destination system.

mynetworks = 127.0.0.0/8
relay_domains = smtp.pangaea.edu

mynetworks = 127.0.0.0/8, 172.24.0.0/16, 192.168.24.0/24
relay_domains = $mydestination, pangaea.edu

relayhost = relay.pangaea.edu

transport_maps = hash:/etc/postfix/transport

.subnet1.pangaea.edu  smtp:exchange1.pangaea.edu
subnet1.pangaea.edu   smtp:exchange1.pangaea.edu
.subnet2.pangaea.edu  procmail:/etc/procmailrcs/exchange2
subnet2.pangaea.edu   procmail:/etc/procmailrcs/exchange2