Thinking about this further, the reply code should be `421`, which indicates that the error that occured will also result in the connection being closed. In that case, please merge this with the exception handler above it, like so:

```python
        except (SmtpError, Timeout) as e:
            if not result.ready():
                reply = Reply('421', '4.3.0 {0!s}'.format(e))
                relay_error = SmtpRelayError.factory(reply)
                result.set_exception(relay_error)
```