[Dovecot] How do I use sieve to move bounces?

How can I use sieve to move bounces and non delivery reports (i.e. anything with an empty envelope from) to a special mailbox? I placed this snippet at the start of my sieve script

require ["envelope", "fileinto"]; if envelope :is "from" "" { fileinto "INBOX.Reports"; stop; }

But mails are not moved to the reports mailbox, and they are still processed by the rest of the script.