My postfix logs were reporting that mail sent to non-existent was getting greylisted. I would expect this mail to get rejected right away, so I dug up the reason why it wasn’t. Turns out I only had the virtual domains getting checked, not the mailboxes. So I added this line back in (I had commented it out for simplicity!):

virtual_mailbox_maps = mysql:/etc/postfix/

As explained in the docs, virtual_mailbox_maps is an optional setting:

Recipient address rejected: User unknown in virtual mailbox table;