Install

apt-get install ssmtp

Configure

vi  /etc/ssmtp/ssmtp.conf

#
# Config file for sSMTP sendmail
#
# The person who gets all mail for userids < 1000
# Make this empty to disable rewriting.
root=xxx@yahoo.com

# 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.mail.yahoo.com:465
AuthUser=xxx@yahoo.com
AuthPass=xxx
UseTLS=YES
UseSTARTTLS=NO

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

# The full hostname
hostname=deepthought

# 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

Test

echo "This is a test" | sendmail xxx@yahoo.com

Configure an App Password on Yahoo

You may need to generate an app password on Yahoo in order to send email.

From the Yahoo Mail UI, click your Name and then click on Account Info.

From here, select Account Security and click on Manage App Passwords.

Use the generated password in your email settings.

Configure Cron Jobs

Because cron uses a minimal shell, we need to update it's path. 

crontab -e

Set the path for cron and MAILTO at the beginning of the file.

Setting a MAILTO="" will disable cron from sending emails for jobs which generate output.

PATH=/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin
MAILTO=""

If you want to get cronjob emails, set MAILTO in the crontab:

crontab -e

Set the path for cron and MAILTO at the beginning of the file.

PATH=/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin
MAILTO="xxx@yahoo.com"

You will also need to replace the symlink for sendmail with a script.

rm /usr/sbin/sendmail

vi /usr/sbin/sendmail

#!/bin/sh
ARGS=$@
sed -r -e 's/^From: root \(Cron Daemon\)\s*$/From: Cron Daemon <xxx@yahoo.com>/' | /usr/sbin/ssmtp $@

chmod +x /usr/sbin/sendmail

References