milterの挙動
https://www.milter.org/developers/overview#ControlFlow

  For each of N connections {
    For each filter
      process connection (xxfi_connect)
    For each filter
      process helo (xxfi_helo)
    MESSAGE:For each message in this connection (sequentially) {
      For each filter
        process sender (xxfi_envfrom)
      For each recipient {
        For each filter
          process recipient (xxfi_envrcpt)
      }
      For each filter {
        process DATA (xxfi_data)
        For each header
          process header (xxfi_header)
        process end of headers (xxfi_eoh)
        For each body block
          process this body block (xxfi_body)
        process end of message (xxfi_eom)
      }
    }
    For each filter
      process end of connection (xxfi_close)
  }