Remove installed MTA

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