You could be missing important information about available updates, as Drupal uses cron to sends notification e-mails once updates are available. The following written applies not only to Drupal sites, it basically applies for Nginx and Ubuntu 16.04. Since I was working with a Drupal 8 project when faced unable to send e-mail, I'm mentioning Drupal.
Inspired by: Email not working with fresh Drupal install on Ubuntu 14.04
Anyhow, here is a simple fix, how to quickly enable mail send for your server.
Install Postfix in 3 simple steps
You will need to have access to your VPS, login with user with sudo priviligies
sudo apt-get install postfix
Follow on screen instructions, leaving defaults
Then, edit /etc/postfix/main.cf and set inet_interfaces to loopback-only
inet_interfaces = loopback-only
Finally, restart postfix:
sudo service postfix restart
That's it! You are good to go!
