Ssmtp

Setup SSMTP to work with your gmail account on a Raspberry PI 3 (stretch)

Make sure you have ssmtp installed, if not then sudo apt-get install ssmtp

sudo nano /etc/ssmtp/ssmtp.conf

Edit you ssmtp.conf file to look like this:

#
# Config file for sSMTP sendmail
#
# The person who gets all mail for userids < 1000
# Make this empty to disable rewriting.
root=<the name of your server here>

# The place where the mail goes. The actual machine name is required no
# MX records are consulted. Commonly mailhosts are named mail.domain.com
mailhub=smtp.gmail.com:587

# Where will the mail seem to come from?
#rewriteDomain=

# The full hostname
hostname=<hostname of your serve here>

# Are users allowed to set their own From: address?
# YES - Allow the user to specify their own From: address
# NO - Use the system generated From: address
FromLineOverride=YES
UseSTARTTLS=YES
AuthUser=<yourgmailaccount>@gmail.com
AuthPass=<your app password>

Setup an app password with gmail

https://security.google.com/settings/security/apppasswords