The microservice that sends emails to users builds on the image from the Dockerfile stored in the ./microservices/mails folder. This microservice depends on the smtp container, but this microservice doesn't check that the mail service is ready for work. If you want to check that the mail server is ready, add a piece of code that will try to connect to the SMTP server before starting any activity. Look at the following settings:

mails:
  build: ./microservices/mails
  depends_on:
    - smtp
  environment:
    - RUST_LOG=mails_microservice=debug
    - RUST_BACKTRACE=1
    - MAILS_ADDRESS=0.0.0.0:8000
    - MAILS_SMTP_ADDRESS=smtp:2525
    - [email protected]
    - MAILS_SMTP_PASSWORD=password
  ports:
    - 8002:8000

We also configure a microservice with environment variables and forward port 8002 to port 8000 of the container. You  can use port 8002 to check that the microservice started and works.