Every sending platform reports a delivery rate, and it is almost always above 98%. That number only says that the receiving server accepted the message. It says nothing about where the message went afterwards. A message filed into the spam folder has been "delivered" in every statistic you will ever see, and nobody will ever read it.

Delivery versus inbox placement

Delivery is the moment the receiving mail server answers "250 OK" to your sending server. From that point your provider considers the job done.

Inbox placement is what happens next: the provider's filters decide between the inbox, a tab such as Promotions, the spam folder, or silent discard. This decision is not reported back to you. There is no bounce for spam. The only way to know is to look inside real mailboxes.

Why the gap is so expensive

Imagine a list of 50,000 subscribers at an average 20% open rate. If one large provider starts filing your campaign into spam, you lose a fifth of your audience without a single error in your dashboard. Open rates drift down, revenue drifts down, and the usual explanation is "people are less engaged". The real cause is a filter decision that nobody measured.

The same applies to transactional email. Password resets, invoices and order confirmations that land in spam generate support tickets, not warnings.

What decides placement

Providers combine many signals, and the weighting changes constantly, but the big ones are stable:

  • Authentication: SPF, DKIM and DMARC must pass and must align with the visible From domain.
  • Reputation: the history of your sending domain and IP addresses, including complaints and spam-trap hits.
  • Engagement: whether recipients open, reply, delete without reading or mark as spam.
  • Content and structure: broken HTML, link shorteners, image-only messages and mismatched link domains all raise suspicion.
  • List hygiene: bounces and inactive addresses signal a careless sender.

How to measure it

A seed test sends one message to a set of real mailboxes at the providers you care about and reports, per mailbox, where it landed. It is the closest thing to standing behind your subscribers and watching their screens. Run it before every important campaign, after any change to DNS or sending infrastructure, and on a regular schedule so that you notice a reputation problem while it is still small.

Mailproof does exactly that: copy the seed list, add the identifier, send, and read the placement for every provider within five minutes.