Why delivery rate alone doesn’t tell you about placement
A message can be accepted by Gmail’s servers (counted as “delivered” by your ESP) and still be filtered into spam entirely after acceptance, based on Gmail’s own reputation and content analysis. ESP delivery-rate reporting typically stops at acceptance and has no visibility into what the receiving provider does with the message afterward — that’s specifically the gap inbox placement testing is designed to measure.
How seed-list testing works
A testing service maintains real, monitored email accounts across the major mailbox providers your audience likely uses. A handful of these seed addresses get included in an actual campaign send, and the service then checks each account to see whether the message landed in the inbox, a specific tab, or spam — giving a provider-by-provider placement snapshot for that specific send.
Limitations of seed-list testing
Seed accounts don’t have the same engagement history as your real subscribers, so their individual filtering behavior can differ somewhat from how a long-time, engaged real recipient’s mailbox treats the same message. Seed-list results are a useful directional signal and early-warning system, not a perfect, guaranteed match for every real recipient’s actual experience.
What to do when inbox placement is poor
Poor placement despite acceptable delivery rates usually points to a reputation or authentication issue (check SPF/DKIM/DMARC alignment and recent bounce/complaint history) rather than a list-validity issue, since the message is already being accepted — the problem is what happens to it after acceptance, which list cleaning alone won’t fix if the underlying reputation or authentication problem persists.