We recently had an audit that reported invalid emails are returned rather than configured to Silent Drop as they were last year. Might have been due to a config change from a patch but you should revert back to the Silent Drop.
We have verified that "Allow non-delivery report" was unchecked in Exchange System manager, but the NDR's are still being sent. Does anyone have any ideas what we are missing? We made no major config changes to exchange over the past year.