Non-Delivery Notification For Email I Didn't Send

For almost a year I had have MIAB set up with a couple of domains in addition to the primary. One of the domains also has a website sitting on another vps. The DNS is managed by MIAB and I have the DNS settings for the website pointing to the ip for the second vps. On the web server I have Postfix with my MIAB set as a relay to send and receive email for contact forms, etc.

Yesterday I received a non-delivery notification for an email account associated with this second domain. It’s the email account I use for the website.

Looking over the logs I found: to=<unknown-user@[my-vps-ip-address]

The email bounced but I was surprised it wasn’t instead rejected.

So my question is this - is this the expected response a bounce rather than rejected ? Or do I have a misconfiguration somewhere ?