Qmail

Initial setup
A directory for qmail to reside in needs to be created:

Rather than being a single program, Qmail is a collection of smaller programs doing specific tasks. Each run as their own user to provide further security to your server - those users need to be created.
One thing to note - the numeric UID/GID values used by qmail and vpopmail are hard-coded into the programs when they are compiled. If you backup and restore your mail onto a different server, the UID and GID MUST HAVE THE SAME NUMERIC VALUE ON BOTH SERVERS! The easiest way to ensure this is to manually specify the values when the users and groups are created. Check your /etc/passwd and /etc/groups files first to make sure the numbers given here aren't used. Most Linux systems use numbers higher than 500 for "regular" users and less than 100 for "system" so we've picked a middle ground.

Download Compile Qmail
Qmail, by itself, is lacking some of the capabilities we want on our server. So we'll also download the latest version of John Simpson's Combined Patch which will add some very useful features to our install.

Apply the patch then compile and install qmail:

Configure Qmail

Set up the daemontools service
Create the "container" directory which will contain the service directories for all of the qmail-related services...

Create the service directory structure for the qmail-send service itself:

Then create the log directory and run script:

Finally, start the service running.

After a few seconds, confirm that the service is running:

Create an SSL certificate

• Certificate from LetsEncrypt
  
  cd /var/qmail/control
ln -s /etc/letsencrypt/live/server.domain.com/combined-for-pound.pem servercert.pem
ln -s /etc/letsencrypt/live/server.domain.com/combined-for-pound.pem clientcert.pem
• Self-Signed Certificate
  In order to provide secure, encrypted access to some of the services on this server, we're going to create a self-signed SSL certificate. When creating the certificate, it will ask for a "Common Name" - this must exactly match the name by which clients will connect to your server.
  cd /var/qmail/control

  Create the "servercert.pem" file, which is used to encrypt incoming SMTP connections as needed:

  openssl req -newkey rsa:1024 -nodes -x509 -days 3650 -keyout servercert.pem -out servercert.pem

  Change the permissions to secure the file:

  chown root:nofiles servercert.pem
chmod 640 servercert.pem

  Then make a copy of the key file and change it's group to qmail. The copy will be used by the qmail-remote user for outgoing mail deliveries.

  cd /var/qmail/control
cp servercert.pem clientcert.pem
chown root:qmail clientcert.pem
chmod 640 clientcert.pem

Remove Sendmail

• CentOS 5/6
  Now that qmail is up and running, we'll remove Sendmail from our machine. Find out what the Sendmail package is called:
  rpm -qa | grep sendmail

  Then stop Sendmail and remove the found package. On a CentOS 4 server it was sendmail-8.13.1.3.2.el4 for CentOS 5 it was sendmail-8.13.8-2.el5.

  /etc/rc.d/init.d/sendmail stop
rpm -e --nodeps sendmail-8.13.8-2.el5

  Qmail works as a drop-in replacement for Sendmail, so create some symbolic links for any programs on our system that might use Sendmail:

  ln -s /var/qmail/bin/sendmail /usr/lib/sendmail
ln -s /var/qmail/bin/sendmail /usr/sbin/sendmail
• CentOS 7
  Stop and disable Postfix:
  systemctl stop postfix
systemctl disable postfix

  Find out what the Postfix package is called:

  rpm -qa | grep postfix

  Then stop Sendmail and remove the found package.

  rpm -e --nodeps postfix-2.10.1-6.el7.x86_64

  Add the symlinks:

  ln -s /var/qmail/bin/sendmail /usr/lib/sendmail
ln -s /var/qmail/bin/sendmail /usr/sbin/sendmail

Install the man pages
The man pages are installed in /var/qmail/man You'll need to add that to your MANPATH variable. Edit /etc/man.config and add:

Or export it temporarily with