PREUN

/bin/sh

if [ $1 = 0 ]; then
        update-alternatives --remove sendmail-command /usr/sbin/ssmtp
fi

POSTIN

/bin/sh

update-alternatives --install /usr/sbin/sendmail sendmail-command /usr/sbin/ssmtp 5 --slave /usr/lib/sendmail sendmail-command-in_libdir /usr/sbin/ssmtp