Delivery: a technical, binary outcome
Delivery is determined at the SMTP layer — did the receiving server return a success response (250 OK) and accept the message, or did it bounce (4xx/5xx)? This is what most ESP dashboards report as “delivery rate,” and it’s primarily a function of whether the recipient address and domain are valid and the sender isn’t outright blocked.
Deliverability: what happens after acceptance
Once a message is accepted, the receiving mail server applies spam filtering based on sender reputation, authentication results, content analysis, and engagement history before deciding inbox, spam folder, or (rarely) outright silent discard. A message can clear the delivery bar completely and still fail deliverability by landing somewhere the recipient never sees it.
Why this distinction matters practically
Teams sometimes see a healthy “delivery rate” from their ESP and conclude everything is fine, while open rates quietly decline because an increasing share of “delivered” mail is landing in spam. Monitoring actual inbox placement (via seed-list testing tools or mailbox-provider postmaster tools) catches this gap that delivery-rate metrics alone miss entirely.
How list hygiene affects both, differently
Invalid addresses hurt delivery rate directly (they bounce). But a list full of technically-valid, rarely-engaged addresses can hurt deliverability indirectly even with a perfect delivery rate — low engagement is itself a negative signal to mailbox providers, which is why list hygiene is about more than just removing addresses that will bounce.