If you get this message, after the installation of Exim4 :
ALERT: exim paniclog /var/log/exim4/paniclog has non-zero size, mail system might be broken
Try these commands :
sudo apt-get remove --auto-remove sendmail
There is an incompatibility between sendmail and Exim4 as they use the same port (port 25).
sudo service apache2 restart
If you still get it :
Remove all the lines. Ctrl+O, Ctrl+X.