Whether the server has a "default email address" depends entirely on its configuration.

E.g. if the server does not have an SMTP server running, the server will not accept any mail being sent to it, period.

If you want to do the "good" thing (in my opinion), you need to configure an email address (not necessarily on that server) to which bounced and/or replied email will be returned. And particularly, have a real person monitor the "replied" mails (possibly with a filter to filter out bounces to Reply-To). As responsible people, you (or your employer / client) should know if someone e.g. does not wish to receive any more emails from your service and act on it.

Liz