$ apt-get install ssmtp

$ vi /etc/ssmtp/ssmtp.conf
root=sender.email@gmail.com
mailhub=smtp.gmail.com:587
rewriteDomain=
hostname=mycomputerName
UseTLS=YES
UseSTARTTLS=YES
AuthMethod=LOGIN
AuthUser=sender.email@gmail.com
AuthPass=sender.email.password
FromLineOverride=YES

$ chmod 640 /etc/ssmtp/ssmtp.conf

$ ssmtp username@gmail.com
Hi
Ctrl+D
ssmtp: Cannot open smtp.gmail.com:587

root=myemail@gmail.com
mailhub=smtp.gmail.com:587
rewriteDomain=gmail.com
hostname=YourRaspberryPiHostName
UseTLS=YES
UseSTARTTLS=YES
AuthUser=myemail@gmail.com
AuthPass=passwordofmyaccount
AuthMethod=LOGIN
FromLineOverride=YES