Support RFC3463 Enhanced Mail System Status Codes

[d--j]

Where the milter defines the SMTP response (e.g. https://pkg.go.dev/github.com/d--j/go-milter#RejectWithCodeAndReason ) we should also add support for enhanced status codes.

You can of course prepend the enhanced status code manually but since we might re-wrap the status text automatically we should also handle the enhanced status code while re-wrapping the text.