Every team that shares an IP pool shares each other’s mistakes. Reputation is a commons, and commons get overgrazed.
We measured cross-team contamination across four quarters: one bulk sender’s complaint spike cost transactional traffic a measurable delivery dip for six days.
The fix is not policy documents. It is per-domain routing rules and breakers that isolate the grazing before the field burns.
The mechanism is boring, which is the point. Marketing blasts ride the marketing vendor’s reputation; password resets and receipts ride their own lanes. When a complaint spike hits, the breaker opens on the offending domain only — the transactional lane keeps its 99.2% delivery rate while the grazers are fenced off to recover. No email to the whole company. No naming and shaming. Just routing rules that make the commons behave like it is owned, because it is.
The four studies behind this piece all land on the same conclusion: reputation debt is issued silently and collected suddenly. The teams that stay ahead are the ones treating every shared pool as a budget with circuit breakers attached, not a free pasture with a wiki page about best practices.