Exchange2003 is encountering a letter-back spam attack

The first thing you want to do is check out who sent the emails, whether they were sent by the system.
First determine if the problem is spam!

If it is, quite simply, turn off the Exchange2003 NDR,
Now these are sent to the nonexistent user, and then the system sends an NDR to the other.
In fact, these addresses are counterfeit.

According to my settings,
Open System Manager,
Global Settings-Internet Mail format
Advanced-Cancel "Enable Non-delivery report".

Microsoft defaults to this option will cause Exchange2003 to encounter a letter of spam attack.
For example, the garbage manufacturer sends a large number of unknown users to Exchange2003, and the default Exchange sends Non-delivery reports to the masked sender.

The purpose of the garbage manufacturer is to use NDRs to get the fake senders to receive junk mail.

One of the easiest ways to do this is if you're using an NDR. You can enable filtering of recipient filters that are not in the directory to prevent this type of garbage.