ssmtp_gmail.conf

vim /etc/ssmtp/ssmtp.conf

root=emailaddresss
mailhub=smtp.gmail.com:587
AuthUser=emailaddress
AuthPass=XXXXXXXXXXX
UseSTARTTLS=Yes
UseTLS=Yes
hostname=localhost

Save and Exit

vim /etc/ssmtp/revaliases 

add lines 
root:somebody@gmail.com:smtp.gmail.com:587

save and Exit

nano msg.txt

To: MyEmailAddress@gmail.com
From: MyEmailAddress@gmail.com
Subject: test email

Hello World!

Save and exit

/usr/sbin/ssmtp MyEmailAddress@gmail.com < msg.txt

Be sure to set your Gmail email address where it has:
MyEmailAddress@gmail.com