I run docker setup and I had postfix installed on the host, but it didn't work and I just re-setup in configurations.yaml which worked straight away, just another place to update PWD tho....
notify:
- name: NOTIFIER_NAME
platform: smtp
server: smtp.gmail.com
port: 587
timeout: 15
sender: john@gmail.com
encryption: starttls
username: john@gmail.com
password: thePassword
recipient:
- james@gmail.com
- bob@gmail.com
sender_name: My Home Assistant