How do I send mails from a self managed Gitlab server?

Question

Thats a part of my Docker-Compose File. The problem is that the mails dont reach the users. Does someone knows why?

version: '3.6'
services:
  web:
    image: 'gitlab/gitlab-ee:latest'
    restart: always
    hostname: 'rainws02.tjema.local'
    environment:
      GITLAB_OMNIBUS_CONFIG: |
        external_url 'https://rainws02.rain.local'
        gitlab_rails['smtp_enable'] = true
        gitlab_rails['smtp_address'] = "rainex01.rain.local"
        gitlab_rails['smtp_port'] = 465
        gitlab_rails['smtp_user_name'] = "gitlab@rainlocal"
        gitlab_rails['smtp_password'] = "PASSWORD"
        gitlab_rails['smtp_domain'] = "rain.local"
        gitlab_rails['smtp_authentication'] = "login"
        gitlab_rails['smtp_enable_starttls_auto'] = true
        gitlab_rails['smtp_tls'] = false
        gitlab_rails['smtp_openssl_verify_mode'] = 'peer'
        gitlab_rails['gitlab_email_from'] = "[email protected]"
...