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Configuring a POP Server
As a general rule, POP servers are simple to set up and run.
Most require no configuration, aside from that needed to start the server, as
described in Chapter 4 . It's important, though, that you
use a POP server that's suited to your SMTP server. Most importantly, there are
two formats used for storing incoming mail: mbox and maildir. Most mail servers
use mbox by default, but many can use maildir instead, and a few use maildir by
default. Specifically, sendmail, Postfix, and Exim use mbox by default, and
qmail uses the maildir format by default. Postfix, Exim, and qmail can all be
configured to use either mbox or maildir. Your POP server must be able to read
mail from the appropriate format, and so must understand the format used by
your SMTP server.
POP
Servers for Linux
POP servers ship with every major Linux
distribution. It's usually easiest to use the tool that ships with your
distribution, but if you've replaced or reconfigured your SMTP server to use an
unusual incoming mailbox location or format, you may need to replace the POP
server to match the new format. POP servers you might use include the
following: UW IMAP The University of Washington IMAP server (
ships with many Linux distributions, and uses the mbox format that's the
default with most Linux SMTP servers. Cyrus IMAP Like UW IMAP, Cyrus IMAP (
own mailbox format for folders, and mbox for incoming mail. nupop This server ( target="_blank">http://nupop.sourceforge.net ) was written with efficiency for high-volume sites in mind, so you
may want to look into it if your server must handle mail for many users. It
requires mail to be stored in the maildir format favored by qmail. Courier The Courier server ( target="_blank">http://www.courier-mta.org ) is an integrated POP/IMAP/SMTP mail server. Its POP and IMAP
servers are available in a separate package, Courier-IMAP (
earlier are commercial POP3 servers, but version 4.0 is open source. QPopper
works with the mbox format that's most common on Linux systems. Version 4.0
includes SSL encryption, should you wish to experiment with it. qmail-pop3d This program comes with the qmail mail server (
starting point if you choose to use qmail for handling SMTP.The preceding list is only a beginning; a
search on Sourceforge ( target="_blank">http://www.sourceforge.net ) or a Web search will turn up more POP servers, some of which are
part of integrated packages that support IMAP, SMTP, and even other protocols. Most
Linux distributions ship with UW IMAP, and some also ship with Cyrus, QPopper,
or others.
POP Server Installation and Configuration
Most distributions ship with a POP server in
a package called imap or some variant of that. The POP server usually runs from a super
server, as described in
server may set up a xinetd configuration file to run automatically, or you may need to modify
the /etc/inetd.conf file to have the server run. You'll probably have to restart inetd or xinetd before
the POP server will begin responding to queries. If you install a server that's
not part of your distribution, you should consult its documentation to learn of
any quirks for running it.The default UW IMAP, and most other POP
servers, rely on Linux's normal user database for authentication. Thus, you
shouldn't need to do anything special to configure the server for users on your
system; so long as a user has a regular account, and so long as that account
can receive push mail via an SMTP server, the POP server should be able to
deliver mail to the user. Users must enter their normal login passwords in
their POP mail clients in order to retrieve mail. Because the jobs these
servers do is so simple, they often have no configuration files, unless you
count entries in inetd or xinetd configuration files.
