This should help you get Sendmail installed with basic configuration on Ubuntu.

1. If sendmail isn't installed, install it: sudo apt-get install sendmail
2. Configure /etc/hosts file: nano /etc/hosts
3. Make sure the line looks like this: 127.0.0.1 localhost yourhostname
4. Run Sendmail's config and answer 'Y' to everything: sudo sendmailconfig
5. Restart apache sudo service apache2 restart