Chapter 28

Chapter 28 Configuring Mail Server

E-mail is one of the most useful features of the Internet. For an end user using e-mail is a fast and economical way of sending messages to any distance. It is free and very easy to use. But for an administrator configuring and maintain an e-mail server is a challenging job. Before going to the configuration details let us see some basic facts about the e-mail

How e-mail is Sent and Received Simple Mail Transport Protocol (SMTP) is the protocol used to send mails form one network to another. Email is sent in plain text using SMPT. A Mail Transport Agent (MTA) is used to send and receive mails between networks. It is the duty of the MTA to receive the mail from the user and analyze the address and send it the correct location and from there to the correct user. Some of the famous MTAs are Sendmail, Qmail, Postfix. The Mail User Agent ( MUA), or mail client allows the user to read and compose mails. An end user will be only familiar with this application. Commonly used mail clients are Netscape Messenger, Outlook, pine, mutt, Mozilla Mail, kmail and Evolution

The Mail Delivery Agent (MDA) is similar to an MTA, but does not handle deliveries between systems and does not provide an interface to the user like an MUA. Example would be Procmail or Spamassassin; both provide filtering service to MTA.

Sendmail

Sendmail is one of the most famous MTAs used in the Internet. It is one of the oldest and most powerful MTA exists. The default configuration of the sendmail will work for most basic installations, and only minor changes are needed to configure it for a basic server installation. Some shell scripts are available in the Internet, which will help you to configure the Sendmail easily.

Basic Sendmail Configuration Sendmail configuration is handled by files in the /etc/mail. Most of the configurations are stored in a file sendmail.cf file. The format of this file is somewhat complicated. To make the configuration of the sendmail easier a file named sendmail.mc is created. This file is comparatively less complicated. You can do the editing on sendmail.mc file and using a M4 macro processor you can create a sendmail.cf file from this.

Configure Masquerading Sometimes you might want to have sendmail masquerade as a host other than the actual hostname of your system. Such a situation could occur if you have a dial-up connection to the Internet and your ISP handles all your mail for you. In this case you will want Sendmail to masquerade as the domain name of your ISP. For example you may add a line in the sendmail.mc file as bellow MASQUERADE_AS (`aita.com’) dnl

Using Smart Hosts If you don’t have a dedicated connection to the Internet, you will probably want to have Sendmail send your messages to your ISP’s mail server and let it handle delivery for you. Without a full time internet connection, you could find it difficult to deliver messages to some locations. In those situations, you can configure Sendmail to function as a smart host by passing email on to another sender rather that attempting to deliver the email directly. To enable a smart host you can add a line with your sendmail.mc file as follows: define (`SMART_HOTS’, `dsl.aita.com’)

Setting message delivery intervals By default Sendmail attempts to deliver messages as soon as it receives them. If you don’t have a full time connection with the Internet, you may need Sendmail to hold all the messages in a queue and attempt to deliver them at a specific time intervals or at your prompt. You can configure sendmail to do so by adding the following lines with your sendmail.mc file. Define (`conDELIVERY_MODE’, `d’)dnl # sendmail –bd –q30m

Building the sendmail.cf file After you have made all your changes to sendmail.mc, you need to rebuild the sendmail.cf file. First, back up your old file: #mv/etc/mail/sendmail.cf /etc/mail/sendmail.cf.old You must run sendmail.mc through the m4 macro processor in order to generate a usable configuration file. A command such as the following, is used to do this: #m4 /etc/mail/sendmail.mc /etc/mail/sendmail.cf

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This command creates the sendmail.cf from the sendmail.mc, which you have modified accordingly.

Mail Relaying

By default, sendmail won’t relay mail that didn’t originate from the local domain. This means that if a Sendmail installation running at aita.com receives mails intended for accelicim.com, and that mail didn’t originate from aita.com, the mail will be rejected and won’t be relayed. If you want to allow selected domain to relay through you add an entry for the domain to the file /etc/relay.domains. If the file doesn’t exists, simply create it in your favorite text editor and add a line containing the name of the domain that you want to allow to relay through you. Sendmail will need to be restarted for this change to take effect

Forwarding Email with aliases. Aliases allow you to have an infinite number of valid recipient address on your system, without having to worry about creating accounts or other support files for each address. For example, most systems have “postmaster” defined as a valid recipient, yet don’t have an actual login account named “postmaster”. Aliases are configures in the file /etc/aliases. Here is an example on an alias entry: postmaster: root

This entry will forward any mail received for “postmaster” to the root user. By default, almost all the aliases listed in the /etc/aliases file forwarded to root. Any time you make a change to root the /etc/aliases file, you will need to rebuild the aliases database before that change will take effect. This is done with # newaliases.

Rejecting Email from specified sites

In Internet some sites are there which will continuously send you large number of unwanted mails. This is generally called spams. You may need to reject mails coming from these types of sites. You can use /etc/access file to automatically reject mails from certain sites. After changing the /etc/access file you have to restart sendmail as follows: # service sendmail restart

Fetchmail Configuration If you are using a dial-up connection to connect with the Internet, you may need some tools to retrieve mails from remote e-mail server to your local server. These tools will use POP3 or IMAP protocol to retrieve mails from the remote mail server and store it in your local server. Fetchmail is a utility used to retrieve mails from a remote mail server and store it in your local server. By default Redhat linux comes with the Fetchmail package.

For configuring fetchmail the first thing you want to do is create a .fetchmailrc file in your home directory. This file will provide the configuration for the Fetchmail. A sample entry in the .fetchmailrc is given below: poll "mail.accelstl.com" proto pop3 user "m.mano" with pass “flower" is "m.mano" here forcecr smtpaddress localhost You can have multiple entries for different users by changing the server name, user name, password and local user name. After creating the fetchmail file you can invoke fetchmail for retrieving mails as follows: #fetchmail –a