To save privacy, I put KNOWN_USER@CORRECT_DOMAIN as a valid email address,
and UNKNOWN_USER@CORRECT_DOMAIN as a non-existent email address, even if
the domain is correct and is equal in both addresses.
Do you really want to make it difficult for people to help you?
Of the three common possibilities that I mentioned before, that looks an
awful lot like the sort of thing that you'd see from a backup MX that
doesn't check users. Is it?
If so, I recommend dropping the backup MX. They're not useful, and they
cause this exact problem, among others.
If you insist on a backup mx, set up your domain as a "hosteddomain" and
write a script that builds a list of all of the valid recipients of that
domain and converts it into an alias file, directing mail to the primary MX.
That's an entirely different issue, and as Marcin pointed out, more
difficult to solve. I see those from time to time, but they're pretty
rare. I expect that the former messages make up the bulk of the
deliveries to your postmaster account.