How can I send mails from the command line?
The first thing you'll need is a gmail account, that is dedicated to this purpose. So go ahead and create one. Then what you need to do is to also allow "less secure apps" to send e-mails. You can do that at https://myaccount.google.com/lesssecureapps.
After having set up your Gmail Account for sending mails, you can log in to your server and run through the following steps.
sudo apt-get install ssmtp mailutils
Open the following file in your favorite text editor (vim, nano, ...):
Append the following lines:
email@example.com mailhub=smtp.gmail.com:465 rewriteDomain=gmail.com AuthUserfirstname.lastname@example.org AuthPass=password FromLineOverride=YES UseTLS=YES
Finally, send mails like this:
echo "Hi there!" | mail -s "My Subject" email@example.com