I incorrectly set up a new group so that any inbound external emails sent to it bounced with "We're writing to let you know that the group you tried to contact (<groupname>) may not exist, or you may not have permission to post messages to the group." Unfortunately this was a job application alias so I'm afraid that interested candidates might have received bounce messages.
Does Google Workspace log inbound emails that bounced during a time window?
I believe you can find them in the Email Log Search under Reporting in the Admin Console. I was just troubleshooting something similar myself and it shows messages that get rejected by Google and never make it to the intended users inbox.