~/fig/nullmailer/
├── Dockerfile
└── nullmailer/
    ├── adminaddr --> username@gmail.com
    └── remotes   --> smtp.gmail.com smtp --port=465 --auth-login --user=username --pass=****** --ssl

Read this to config.

docker-compose.yml

nullmailer:
  image: vimagick/nullmailer
  cotainer_name: nullmailer
  volumes:
    - ./nullmailer:/etc/nullmailer
  restart: always

Up and Running

# start service
$ cd ~/fig/nullmailer
$ docker-compose up -d

# send email (from gmail to yahoo)
$ alias sendmail='docker exec -i nullmailer sendmail -f username@gmail.com'
$ echo -e 'Subject: hello\r\n\r\nworld' | sendmail username@yahoo.com