Install qmail.  QMail is available as standard Debian/Ubuntu package, rpms for Fedora/Redhat/CenTOS are available from QMail wiki page

Replace sendmail with qmail's wrapper.

[root qmail-1.03]# rm -f /usr/bin/sendmail /usr/sbin/sendmail
[root qmail-1.03]# ln -s /var/qmail/bin/sendmail /usr/sbin/sendmail
[root qmail-1.03]#
rm -f /usr/bin/sendmail /usr/sbin/sendmail
ln -s /var/qmail/bin/sendmail /usr/sbin/sendmail

Configure qmail - specifically, run the config script to set up files in /var/qmail/control specifying the computer's identity and which addresses it should accept mail for. This command will automatically set up qmail correctly if you have correctly set a valid host nome. If not, you'll want to read /var/qmail/doc/INSTALL.ctl to find out how to configure qmail.

[root qmail-1.03]# ./config-fast yourserver.test
Your fully qualified host name is yourserver.test.
Putting yourserver.test into control/me...
Putting yourserver.test into control/defaultdomain...
Putting yourserver.test into control/plusdomain...
Putting yourserver.test into control/locals...
Putting yourserver.test into control/rcpthosts...
Now qmail will refuse to accept SMTP messages except to yourserver.test.
Make sure to change rcpthosts if you add hosts to locals or virtualdomains!
[root qmail-1.03]#
./config-fast yourserver.test

All incoming mail that isn't for a specific user is handled by the alias user. This includes all root mail. These commands prepare the alias user to receive mail.

[root qmail-1.03]# cd ~alias; touch .qmail-postmaster .qmail-mailer-daemon .qmail-root
[root alias]# chmod 644 ~alias/.qmail*
[root alias]# /var/qmail/bin/maildirmake ~alias/Maildir/
[root alias]# chown -R alias.nofiles /var/qmail/alias/Maildir
[root alias]#
cd ~alias; touch .qmail-postmaster .qmail-mailer-daemon .qmail-root 
chmod 644 ~alias/.qmail* 
/var/qmail/bin/maildirmake ~alias/Maildir/ 
chown -R alias.nofiles /var/qmail/alias/Maildir

Configure qmail to use the Maildir delivery format (instead of mbox), and install a version of the qmail startup script modified to use Maildir.

[root alias]# echo "./Maildir" > /var/qmail/bin/.qmail
[root alias]# cp /tmp/openacs-5.9.0/packages/acs-core-docs/www/files/qmail.rc.txt /var/qmail/rc
[root alias]# chmod 755 /var/qmail/rc
[root alias]# 
echo "./Maildir" > /var/qmail/bin/.qmail 
cp /tmp/openacs-5.9.0/packages/acs-core-docs/www/files/qmail.rc.txt /var/qmail/rc 
chmod 755 /var/qmail/rc 

Set up the skeleton directory so that new users will be configured for qmail.

[root root]# /var/qmail/bin/maildirmake /etc/skel/Maildir
[root root]# echo "./Maildir/" > /etc/skel/.qmail
[root root]# 
/var/qmail/bin/maildirmake /etc/skel/Maildir
echo "./Maildir/" > /etc/skel/.qmail

As recommended, we will run qmail with daemontools control files. Create daemontools control directories, set up a daemontools control script, copy the supervise control files, and set permissions. The last line links the control directories to /service, which will cause supervise to detect them and execute the run files, causing qmail to start.

[root root]# mkdir -p /var/qmail/supervise/qmail-send/log
[root root]# mkdir -p /var/qmail/supervise/qmail-smtpd/log
[root root]# mkdir /var/log/qmail
[root root]# chown qmaill /var/log/qmail
[root root]# cp /tmp/openacs-5.9.0/packages/acs-core-docs/www/files/qmailctl.txt /var/qmail/bin/qmailctl
[root root]# chmod 755 /var/qmail/bin/qmailctl
[root root]# ln -s /var/qmail/bin/qmailctl /usr/bin
[root root]# cp /tmp/openacs-5.9.0/packages/acs-core-docs/www/files/qmail-send-run.txt /var/qmail/supervise/qmail-send/run 
[root root]# cp /tmp/openacs-5.9.0/packages/acs-core-docs/www/files/qmail-send-log-run.txt /var/qmail/supervise/qmail-send/log/run
[root root]# cp /tmp/openacs-5.9.0/packages/acs-core-docs/www/files/qmail-smtpd-run.txt /var/qmail/supervise/qmail-smtpd/run
[root root]# cp /tmp/openacs-5.9.0/packages/acs-core-docs/www/files/qmail-smtpd-log-run.txt /var/qmail/supervise/qmail-smtpd/log/run
[root root]# chmod 755 /var/qmail/supervise/qmail-send/run
[root root]# chmod 755 /var/qmail/supervise/qmail-send/log/run
[root root]# chmod 755 /var/qmail/supervise/qmail-smtpd/run
[root root]# chmod 755 /var/qmail/supervise/qmail-smtpd/log/run
[root root]# ln -s /var/qmail/supervise/qmail-send /var/qmail/supervise/qmail-smtpd /service
[root root]# ln -s /var/qmail/supervise/qmail-send /var/qmail/supervise/qmail-smtpd /service
mkdir -p /var/qmail/supervise/qmail-send/log
mkdir -p /var/qmail/supervise/qmail-smtpd/log
mkdir /var/log/qmail
chown qmaill /var/log/qmail
cp /tmp/openacs-5.9.0/packages/acs-core-docs/www/files/qmailctl.txt /var/qmail/bin/qmailctl
chmod 755 /var/qmail/bin/qmailctl
ln -s /var/qmail/bin/qmailctl /usr/bin
cp /tmp/openacs-5.9.0/packages/acs-core-docs/www/files/qmail-send-run.txt /var/qmail/supervise/qmail-send/run
cp /tmp/openacs-5.9.0/packages/acs-core-docs/www/files/qmail-send-log-run.txt /var/qmail/supervise/qmail-send/log/run
cp /tmp/openacs-5.9.0/packages/acs-core-docs/www/files/qmail-smtpd-run.txt /var/qmail/supervise/qmail-smtpd/run
cp /tmp/openacs-5.9.0/packages/acs-core-docs/www/files/qmail-smtpd-log-run.txt /var/qmail/supervise/qmail-smtpd/log/run
chmod 755 /var/qmail/supervise/qmail-send/run
chmod 755 /var/qmail/supervise/qmail-send/log/run
chmod 755 /var/qmail/supervise/qmail-smtpd/run
chmod 755 /var/qmail/supervise/qmail-smtpd/log/run
ln -s /var/qmail/supervise/qmail-send /var/qmail/supervise/qmail-smtpd /service

Wait ten seconds or so, and then verify that the four qmail processes are running. If uptimes don't rise above 1 second, this may indicate broken scripts that are continuously restarting. In that case, start debugging by checking permissions.

[root root]# qmailctl stat
/service/qmail-send: up (pid 32700) 430 seconds
/service/qmail-send/log: up (pid 32701) 430 seconds
/service/qmail-smtpd: up (pid 32704) 430 seconds
/service/qmail-smtpd/log: up (pid 32705) 430 seconds
messages in queue: 0
messages in queue but not yet preprocessed: 0
[root root]#

Further verify by sending and receiving email. Incoming mail for root is stored in /var/qmail/alias/Maildir.