Ensure that you run a ispconfig update with reconfiguring services after you reinstalled postfix and dovvecot, otherwise the services are not configured and can not work.
Code:
cd /tmp
wget http://www.ispconfig.org/downloads/ISPConfig-3-stable.tar.gz
tar xvfz ISPConfig-3-stable.tar.gz
cd ispconfig3_install/install
php -q update.php