Here’s a couple of my smart folders that I use at work to help corral emails:
The “Weekly” catch-all:
Receive day is this week | Include Sent Messages
– I have threading turned on for this mailbox to get what is basically the Gmail full email thread feature in Mail
The Flagged for Action:
Captures any email I have flagged as a “actionable” email (using MailTags and Mail’s built-in flag feature) – this is a gathering area for anything that is related to projects I’m currently working on.
The archive mailbox
I use this to grab anything that’s older than 90 days. Every once in a while I go into here and archive everything to a separate mailbox to get it out of the way (but still searchable).