since this is a deep down system’s dependency it will most likely fail on normal operations.
try installing exim4, it will fail
# apt-get install exim4
run and abort to see which packages would be uninstalled
# apt-get purge mail*
hard uninstall of sendmail
# dpkg --force-all -r sendmail-cf # dpkg --force-all -r sendmail-bin # dpkg --force-all -r sendmail-base
try apt-get -f install or directly, exim4 with all dependencies
# apt-get install exim4 exim4-daemon-light exim4-base exim4-config
see how it works again.