Red Hat Enterprise Linux 5 Administration Unleashed

374CHAPTER 18Setting Up an Email Server with SendmailUsing POP and IMAPAfter the emails arrive on the server, users can retrieve them with an email client such asEvolution or Thunderbird. While some email clients can be configured to read emaildirectly from the user’s spool file in /var/spool/mail/, this can be inconvenient becausethe mail client must be run on the email server and the emails have not been runthrough any filters to sort them into easier-to-manage email folders. All emails are aggregatedinto one file, and when the number of emails becomes large (greater than 1,000),reading email directly from the mail spool can be inefficient. Two popular protocols forretrieving email are POP and IMAP.The latest version of POP, or Post Office Protocol, is pop3. It works by “popping” theemail messages off the user’s spool on the email server and saving them in folders on theuser’s local system running the email client. By default, the email is deleted from theemail server after it is saved locally. Optionally, the email client can be configured to filterthe mail into different mail folders before saving them on the local system. By default,emails retrieved using POP are not encrypted, but an encrypted version of pop3 can be setup on the server and used if the email client supports it.Because the email is stored in local mail folders, one disadvantage is that the user mustalways check email from the same computer (locally or remotely). Because the email isremoved from the email server by default (clients can be configured to leave the email onthe server even after it is copied locally), a disk failure on the user’s computer can resultin loss of email unless the email folders are part of a routine backup plan.IMAP, or Internet Message Access Protocol, can also be used to retrieve email from a server.However, unlike POP, the messages are kept on the server, including the email folders usedto organize the messages. The main advantage of IMAP is that users can open any emailclient with IMAP support on any computer that has access to the email server and see alltheir email, complete with email folders and sent mail. Because all the email is stored on theserver, the client is simply used to allow the user to read email off the server. Messages canoptionally be copied to the user’s local system so they can be read while not connected tothe network, but the email is still kept on the server unless specifically deleted by the user.By default, the IMAP connection is not encrypted, but an encrypted version of IMAP can beconfigured on the server. IMAP also simplifies an administrator’s backup procedure becausehe only needs to back up the directories that store email on the email server instead of oneach user’s computer. If a user has a disk failure on her computer, she can log in to anothercomputer and continue reading her email while the disk failure is fixed.Enabling POP and IMAPAfter setting up Sendmail on the email server, install the dovecot RPM package to set upthe IMAP and/or POP protocols. Refer to Chapter 3, “Operating System Updates,” fordetails on how to install an RPM package.In /etc/dovecot.conf find the following line and uncomment it (lines that begin with #are comments):protocols = imap imaps pop3 pop3s