How do you set a rule to move sent items in Mail?

I've got a rule set in my Apple Mail app to move email older than 90 days out of my inbox to a separate local mailbox (to free up some space on my server – I use imap). My inbox rule is If [all] of the following conditions are met: Account [My Main Account] Date Received / Is Greater Than / 90 days Perform the following action Move Message to mailbox: [My Local Backup Mailbox] That works well, but I want to do the same for sent items. I can't see any rule options that affect the Sent items folder (or any other specific mailbox for that matter) Any idea? I'm on El Capitan 10.11.6