As was explained above, filtering just sets up addresses for delivery - no deliveries are actually done while a filter file is active. If any of the generated addresses subsequently suffers a delivery failure, an error message is generated in the normal way. However, if the filter command which sets up a delivery is preceded by the word noerror, errors for that delivery, and any deliveries consequent on it (that is, from alias, forwarding, or filter files it invokes) are ignored.