Guide updated on 29th of February 2016

Remove installed MTA

Exim is the default installed MTA on Debian.

Stop and remove Exim
service exim4 stop
dpkg --ignore-depends=exim4 -r exim4
dpkg --ignore-depends=exim4-daemon-light -r exim4-daemon-light

If you are using sendmail or postfix :
dpkg --purge --ignore-depends=postfix postfix
dpkg --purge sendmail sendmail-base sendmail-bin sendmail-cf

Install a pseudo MTA (mta-local_1.0_all.deb) to avoid problem within Debian system
dpkg -i /downloads/deb-packages/mta-local_1.0_all.deb

Create Symlink to use Qmail instead of the default MTA
rm -f /usr/lib/sendmail
rm -f /usr/sbin/sendmail
ln -s /var/qmail/bin/sendmail /usr/lib/sendmail
ln -s /var/qmail/bin/sendmail /usr/sbin/sendmail

Configure Qmail

Users comments
Color Coded Qmail Installation Key
  Regular Black Text     Qmail installation notes and summaries by the author.
  Bold Black Text     Commands to be run by you, the installer.
  Bold/Regular Red Text    Vital and/or critical information.
  Regular Blue text     Denotes helpful tips and hints or hyperlinks.
  Regular Orange Text     Command line output.
  Bold/Regular green text     Denotes the contents of a file or script.