use Postfix instead and carry on working correctly (because Postfix uses a
Sendmail wrapper, which deceives other programs into thinking that Postfix
is Sendmail). This wrapper—or, more correctly, interface—makes switching
to Postfix extremely easy if you are already running Sendmail. Postfix also
happens to be the MTA of choice for Ubuntu, so it is this one that we spend
more time on later in this chapter.
For enhanced security, many Postfix processes used to use the chroot
facility (which restricts access to only specific parts of the file system) for
improved security, and there are no setuid components in Postfix. In
Ubuntu, a chroot configuration is no longer used and is, in fact,
discouraged by the Postfix author. You can manually reconfigure Postfix to a
chroot configuration, but that is no longer supported by Ubuntu.
If you are starting from scratch, Postfix is considered a better choice than
Sendmail.
Qmail and Exim
Qmail is a direct competitor to Postfix but is not provided with Ubuntu.
Qmail is designed to be easier to use than Sendmail, as well as faster and
more secure. However, Qmail is not a drop-in replacement for Sendmail, and
migrating an existing Sendmail installation to Qmail is not quite as simple as
migrating from Sendmail to Postfix. Qmail is relatively easy to administer,
and it integrates with a number of software add-ons, including web mail
systems and POP3 servers. Qmail is available from www.qmail.org.
Exim is yet another MTA, and it is available at www.exim.org. Exim is
considered faster and more secure than Sendmail or Postfix but is quite
different to configure than either of those. Exim and Qmail use the maildir
format rather than mbox, so both are considered “NFS safe” (see the
following sidebar).
MAILDIR VERSUS MBOX
Qmail introduced maildir, which is an alternative to the standard UNIX
method of storing incoming mail. maildir is a more versatile system of
handling incoming email, but it requires your email clients to be
reconfigured, and it is not compatible with the traditional UNIX way of
storing incoming mail. You need to use mail programs that recognize the
maildir format (which modern programs do).
The traditional mbox format keeps all mail assigned to a folder
concatenated as a single file and maintains an index of individual emails.