This should help you get Sendmail installed with basic configuration on Ubuntu.
- If sendmail isn't installed, install it: 
sudo apt-get install sendmail - Configure 
/etc/hostsfile:nano /etc/hosts - Make sure the line looks like this: 
127.0.0.1 localhost yourhostname - Run Sendmail's config and answer 'Y' to everything: 
sudo sendmailconfig - Restart apache 
sudo service apache2 restart 
For this I think you'll also have to allow gmail on less secure app https://support.google.com/accounts/answer/6010255?hl=en