CentOS - Mail Server - Basic Settings in main.cf

Following from the Postfix installation article, we can now look at the main.cf file and see what the settings are and what they mean.

Note that these settings are intended for a multiple domain configuration of your mail server.


Modular

One of the key aspects of understanding and administering postfix is that it is designed to be a modular package.

By that, I mean that the base installation itself is fairly small and the vast majority of the 'usual' mail administration, such as anti-spam and anti-virus, are actually conducted by third party packages like SpamAssassin .

Although those particular aspects are for a later article, we can begin to see the modular nature of Postfix when we look at the main.cf file.

Many settings refer to other files on the Slice. This setup can, at first, seem slightly confusing and the initial reaction may be to hard code the data rather than reference another file with a single word in it.

I would advise sticking with the modular premise of Postfix and editing multiple files for what may seem like one simple setting.

It makes a lot more sense when we start adding multiple domains, users and aliases to our setup.

Main.cf

So what is this main.cf file?

Let's take a look:

sudo nano /etc/postfix/main.cf

I won't paste it here as we are only going to look at a few settings.

The settings we want to look at are as follows on the demo slice:

#myhostname = host.domain.tld

alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases

#myorigin = $myhostname

#mynetworks = 168.100.189.0/28, 127.0.0.0/8
inet_interfaces = localhost

#home_mailbox = Maildir/

Settings

Although some of the settings may be self explanatory, let's go through some of them so we have a better understanding of the nature of postfix and what we can do with it at this early stage.

MyHostname

This setting is used to specify the fully-qualified domain name of the slice. By default, the myhostname setting will use the hostname of the slice. As discussed earlier, Postfix is modular in nature and many settings do not need to be hard coded.

With that said, we are going to explicitly define the hostname as many other settings rely on this to option to be set correctly.

myhostname = mail.demoslice.com

Good.

This will allow us to ensure that no unexpected issues arise as a result of the hostname setting.

Aliases

Aliases are ways of delivering mail to different users without having to set up dozens of different accounts.

The default settings in the main.cf are good and reference another file:

alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases

Have a look at the file:

sudo nano /etc/aliases

You will see a list of names followed by 'root'. In these instances, mail delivered to the first name will actually be delivered to the second name.

We don't need to setup the postmaster, news, webmaster, abuse, etc users for Postfix as mail delivered to those names will be sent to root.

Using the same syntax, we can have all mail for root delivered to our admin user by adding this line:

root: demo

Remember, 'demo' is the main admin user for this Slice.

You may notice that there will be up to three changes in delivery destination:

Mail sent to 'mailer-daemon' is sent to postmaster.

Mail to postmaster is sent to root, and we have just added that all mail sent to root is sent to the main admin user 'demo'.

You are, of course, free to adjust the aliases as you see fit, but instead of changing all the 'root' users in the file, it is easier and quicker to add the one line as shown above - this also makes for easier migration and administration at a later date.

If you have changed the aliases file you must then refresh the aliases database or any changes will not be affected:

sudo newaliases

MyOrigin

This setting is important as internal emails from packages such as cron jobs do not supply full mail 'credentials' such as sender email. Instead, they rely on 'myorigin' setting. As such, it needs to be set to the main hostname of the Slice.

By default, the setting refers to the $myhostname parameter. However, there is another way of setting 'myorigin' and that is to use the '$mydomain' option in the main.cf file like this:

myorigin = $mydomain

We haven't specifically set the $mydomain variable at any point but postfix gets the information from the 'myhostname' setting - parsing the hostname to gain the main domain name.

The advantage of setting 'myorigin' as shown is that it will make for easier administration going forward. Changes to only one setting (myhostname) will automatically change the other settings as well.

MyNetworks

This setting defines the network that Postfix will allow to send mail. If configured incorrectly, it could allow your mail server to be used as an open relay, which allows unauthorized users to send emails through your slice.

Set this parameter as shown below to only allow your slice to send mail:

mynetworks = 127.0.0.0/8

Inet_Interfaces

This setting is used to define the interface that Postfix sends and receives mail on.

The default setting is localhost, so we need to change this to listen on all interfaces.

inet_interfaces = all

Maildir

Finally, we need to specify the format for our mailbox. The default is to place mail in one file located in /var/spool/mail/user.

We want to clean this up a little bit in favor of a qmail-style delivery for our messages.

Therefore, we need to uncomment the following line:

home_mailbox = Maildir/

The Rest

The remaining settings can be left unchanged at this stage.

They will come into play later on when we look at some more complex configurations but, for the moment, they are not needed and can be left at the defaults.

Final Settings

After the changes we have made, the settings we discussed in this article should look like this in the main.cf file:

myhostname = mail.demoslice.com

alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases

myorigin = $mydomain

mynetworks = 127.0.0.0/8

inet_interfaces = all

home_mailbox = Maildir/

All done.

Now we can move onto restarting Postfix.

Restart

As with all packages, once you have made any changes to the configuration, you will need to restart it:

sudo /etc/init.d/postfix restart

Once that is done, we can conduct a quick test.

Send Mail

As with all administrative changes (not that we made many changes here), it is always a good idea to test them.

Send mail to a working email address:

mail user@example.com
Subject: test
test
.
Cc:

You should receive an email from the correct user and the correct domain - check the headers to see if they are correct.

Summary

Getting involved in the configuration of Postfix can be a daunting task.

This introduction should help with the basics, showing that Postfix is simple in its approach and design, and how using variables instead of hard coding can save time and effort in future administration.

Article Comments:

Denis Forcier commented Thu Jan 28 13:47:01 UTC 2010:

Hi: In your article it states that the mynetworks settings should be mynetworks = 127.0.0.0/8 However towards the end you mentioning the final settings for mynetworks is mynetworks = 168.100.189.0/28, 127.0.0.0/8

What is the correct setting - I'm having a bit of a problem getting this to work...BUT postfix still restarts without any problems.

Jered commented Thu Jan 28 15:42:51 UTC 2010:

Hi Denis. The correct setting is the first, "mynetworks = 127.0.0.0/8" - the article has been corrected to reflect that. If you still have problems after that change, post here, or in the forums, or hit Slicehost support chat.

Marty commented Mon Apr 04 18:30:29 UTC 2011:

Following all the Centos postfix steps up to this point and when I send mail at the end of this article I receive it at the other end as from username@com.com instead of username@domain.com.

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