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Unfortunately, it doesn't work when there's a Null Sender in the envelope header. Looking at a spamtrap mailbox, 5 out of the 7 spam messages that came in this morning have a null sender.
Another problem with this not working is that it doesn't provide a delay for the RBLs to kick in. Often the spammer's IP gets listed on the RBL, but it takes a few minutes and greylisting provides a short delay for this to work.