How is inbox placement tested (seed lists, panels)?

Inbox placement is usually tested in two ways.

Seed list testing uses small lists of test addresses placed across many mailbox providers. When you send a campaign to the seed list the testing tool checks where each message lands.

Panel testing uses real subscribers who allow their inbox behavior to be anonymously measured. This shows how your messages perform in real world inboxes rather than controlled test addresses.

Both methods help reveal how mailbox providers classify your content.