How to set domain value in Email -> Server Settings

It’s probably set in your container configuration file, the default contents in the template are:

  DISCOURSE_SMTP_ADDRESS: smtp.example.com
  #DISCOURSE_SMTP_PORT: 587
  DISCOURSE_SMTP_USER_NAME: user@example.com
  DISCOURSE_SMTP_PASSWORD: pa$$word
  #DISCOURSE_SMTP_ENABLE_START_TLS: true           # (optional, default true)
  #DISCOURSE_SMTP_DOMAIN: discourse.example.com    # (required by some providers)
  #DISCOURSE_NOTIFICATION_EMAIL: noreply@discourse.example.com    # (address to send notifications from)