We get this. There is an out of the box rule that prevents an "unknown" user from updating a ticket. Lets say you email a customer distribution group but the customer has changed who is in that distribution list without telling you. If one of the users responds and doesn't have a user record in ServiceNow then the email will be rejected.
There is a setting you can change that will automatically create new user accounts when an email comes in from an unknown. That's OK unless, like us, you have multiple "probes" out there emailing in automated faults. We turned this on in our DEV instance and within a few days we had over 200 new accounts, all belonging to fault nodes.....
We still get the error but we have set up a feedback loop for errored emails so we catch them and then manually process them (e.g. validate the user and, if valid, add the account and then, reprocess the inbound email.
