The example uses SendGrid, you can get your free account here.
First, update your environment with your SENDGRID_API_KEY.
echo "export SENDGRID_API_KEY='YOUR_API_KEY'" > sendgrid.env
echo "sendgrid.env" >> .gitignore
source ./sendgrid.env
mvn package
cd examples
javac -classpath {path_to}/sendgrid-java-http-client-4.0.0-jar.jar:. Example.java && java -classpath {path_to}/sendgrid-java-http-client-4.0.0-jar.jar:. Example
You can do the following to create a .env file:
cp .env_example .env
Then, just add your API Key into your .env file.