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

[Jef Driesen]

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.