Authentication is being requested for anonymous sign-in

For anonymous sign-in:

```swift
app.smtp.configuration.signInMethod = .anonymous
app.smtp.configuration.secure = .startTlsWhenAvailable
```

`AUTH LOGIN` command is send to SMTP server. [Location in source code](https://github.com/Mikroservices/Smtp/blob/2b0db0cac3cfede56ed5c90f22e103f44d3faf49/Sources/Smtp/Handlers/InboundSendEmailHandler.swift#L98).