Ad Waterfall

An ad waterfall (or demand waterfall) is the sequence of demand sources an ad server queries to fill an impression, in priority order. If the highest-priority source (e.g., a direct-sold campaign) fills the impression, the waterfall stops. If not, the server moves to the next tier (e.g., a private marketplace deal), then programmatic demand, then house ads, until the impression is filled or the waterfall is exhausted. In MailAdx's waterfall: Direct-Sold → Programmatic Guaranteed → Direct IO → Addressable → PMP → Open Auction → House Ads. Each tier has a priority score and minimum floor CPM. The waterfall runs at open time — when the subscriber opens the email — ensuring impressions are filled with the highest-paying demand available at that moment.