code: def mail_container(value):
      
      if not re.match(r'.*://.*', value):
          raise VdtTypeError(value)
      mburl = urlparse(value)
      uri_scheme_to_mbclass = {
              'mbox': mailbox.mbox,
              'maildir': mailbox.Maildir,
              'mh': mailbox.MH,
              'babyl': mailbox.Babyl,
              'mmdf': mailbox.MMDF,
          }
      klass = uri_scheme_to_mbclass.get(mburl.scheme)
      if klass:
          return klass(mburl.netloc + mburl.path)
      raise VdtTypeError(value)
docs: Check that the value points to a valid mail container,
      in URI-style, e.g.: `mbox:///home/username/mail/mail.box`.
      `~`-expansion will work, e.g.: `mbox://~/mail/mail.box`.
      The value is cast to a :class:`mailbox.Mailbox` object.