autoexpunge

I have the following in 15-mailboxes.config:

mailbox Trash { special_use = \Trash autoexpunge = 30 days }

I thought that would empty the deleted emails after 30 days. However, I find that in .Deleted Messages/cur there are over 18K messages dating back to the 90's. What do I need to set to make the autoexpunge work?

-- Doug