Why Inbox Placement Is the Quiet Margin Lever in Agency Retainers

Deliverability stopped being a send-side technical chore and became a shared-liability problem: one client's purchased list can drag a shared IP pool down and take every retainer on that pool with it. Agencies that treat authentication, list hygiene, and engagement-based sending as a billable diagnostic line protect existing revenue before they chase new logos. The economics favor the operators who can prove inbox placement as a number, because that number is what clients renew against.