Agencies don't have an inbox volume problem. They have an inbox priority problem. The retainer client who's about to renew should not be a peer to the cold lead who emailed once. The invoice question from your top account should not share a queue with a recruiter cold email.
Apple Mail and Gmail give you the same flat list for both. Folders and labels are a workaround, not a solution — they require constant manual sorting and they fall apart the moment a thread crosses a category boundary.
STAMP is built around triage as a first-class action. Every thread arrives tagged with a reason. VIP, Reply needed, Frustrated, Urgent, Invoice. The home view is the seven threads that actually need you today, ordered by signal that's measurable: who they are, how long they've waited, whether the tone has shifted.