Running Linux, 5th Edition - Matthias Kalle Dalheimer [433]
Browser-based email clients such as gmail or GMX are yet another story. They need to operate on a mailbox that is stored on a server; this mailbox could be filled by either POP3 or IMAP, often automatically. These days, it is quite common for browser-based email clients to use IMAP.
If you want more flexibility (which comes at the price of more configuration and maintenance work), you can install an MTA such as Postfix, described in the next section. You will need a program that transports the mail from your provider's POP3 or IMAP server. This program fetches your mail when you ask it to and passes the messages on to the MTA running on your system, which then distributes the mail to the recipients' mail folders. One program that does exactly that is fetchmail, which we cover later in this chapter. Outgoing mail is again sent via SMTP, but with an MTA running on your machine, you can choose not to send the outgoing messages directly to your provider's SMTP server, but rather to your own server, which is provided by the MTA. The MTA then forwards the mail to your provider, which in turn sends it to the recipients. With this setup, you can instruct your MTA to send outgoing mail at certain intervals so that you do not always have to make a dial-up connection.
The third scenario is meant for machines that have a permanent connection to the Internet, either because they are in a network that has a gateway with a permanent connection, or because they are using a leased line to your Internet provider. In this case, you might want to receive mail messages as soon as they arrive at your provider and not have them stored there. This also requires setting up an MTA. Incoming mail will be directed to your SMTP server (i.e., your MTA). Your provider will have to set things up accordingly for this to work.
Of course, there are many more scenarios for using mail, and mixtures between the three mentioned are possible as well. If you are going to set up a mail service for a whole network, you will most certainly want to read the Linux Network Administrator's Guide (O'Reilly) as well as a book about your MTA.
You have a number of software choices for setting up email on a Linux host. We can't describe all the available email solutions, but we do describe some packages that are often used and quite suitable for their respective tasks. Mail programs for end users, such as KMail and Evolution, have already been described in detail in previous chapters. In this chapter we document what we think are the most popular Linux advanced tools at this time: the Postfix mail transport agent and the fetchmail implementation of the POP3 and IMAP protocols. These are relatively simple to configure but provide all the features most users need. In addition, with these tools, you can cover all the scenarios described earlier.
The Postfix MTA
Several MTAs are available for Linux. Historically, the most common MTA on Unix has been sendmail, which has been around for a long time. It is generally considered somewhat more difficult to use than the alternatives, but it is thoroughly documented in the book sendmail, by Bryan Costales with Eric Allman (O'Reilly).
Postfix is a newer MTA, developed by security guru Wietse Venema as a replacement for sendmail. It's designed to be compatible with sendmail but to provide a higher level of security and be easier to configure.
Postfix is a highly flexible and secure piece of software that contains multiple layers of protection against would-be attackers. Postfix was also written with performance in mind, and employs techniques to limit slower activities such as creating new processes and accessing the filesystem. It is one of the easier email packages to configure and administer because it uses straightforward configuration files and simple lookup tables for address rewriting. It is remarkable in that it is simple to use as a basic MTA, yet still able to handle much more complicated environments.
Many Linux distributions have Postfix built in, so you may already have