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:
The preceding list is only a beginning; a search on Sourceforge (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 Chapter 4. The POP 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.