mail.py:
  define variable related to email
  `body` is the template we will use in agent

  for variables that are pre-defined in the same file,
      use single curly bracket for it.
  e.g. {info_url}

  for `username` or other variables that are undefined until msg is received,
  use four curly brackets for both open and close
  e.g. {{{{ username }}}}