I don't think there's a way with Outlook Eggspress to do that. You can select the "from" address you want to use for each message, but that's not the same.
I don't trust OE (with reasons), and use Thunderbird. That doesn't quite do what you want either, but it does reply from the address the message was sent to (with the right settings, which mine has, but don't ask me how to configure that cos I don't remember).