I have now reconfigured the way my mail works to reduce the number of mailboxes I have and allow me to take advantage of the new spam features.
I have created an alias to send postmaster@ messages to one my mailboxes and have added aliases for all the addresses I use which don't have a full mailbox of their own. I've also turned off catchall.
Now, though, I'm left with the default mailbox of firstname.lastname@example.org, which should not receive any genuine messages - just spam as a result of the address being leaked during the recent security issue.
I want to either delete this default mailbox, or direct all inbound mail it receives to a black hole.