Hello BMur, that clarifies things, your topic name says Google workspace and that is a paid-for service from google.
You will want to create a mail filter that automatically discards any messages sent from *SMTPIN_ADDED_MISSING@mx.google.com – This should be added to the From field of creating a mail filter.
The * is important so do not forget to use that when creating your mail filter.
Tap or click here for instructions on how to create a mail filter.