If program exits 111, bouncesaying exits 111, so delivery will be retried later.
If program exits anything else (or does not exist), bouncesaying exits 0, so the rest of .qmail will be processed as usual.
Note that it is not safe for program to fork a child that reads the message in the background.
If program is not supplied, bouncesaying always bounces the message:
|bouncesaying 'This address no longer accepts mail.'
WARNING: If you create a .qmail file to enable bouncesaying, make sure to also add a line specifying delivery to your normal mailbox.