Using apt-get
$ apt-get update
$ apt-get -s install <pkage> <---- if everything is ok, then,
remove the s
Note you may want to use dpkg to purge if you have to do a reinstall.
$ dpkg --purge exim4-base
$ dpkg --purge exim4-config
$ apt-get install exim4
$ dpkg-reconfigure exim4-config

