Inbox placement
Inbox placement is where a delivered email lands in the recipient's account: the main inbox, a tab such as Promotions, or the spam folder.
Delivered does not mean seen. A message can be accepted by the receiving server and still go to the spam folder, or to a tab like Promotions in Gmail. Inbox placement describes where it ended up. Your sending tool cannot see this from the SMTP reply, because the server says 'accepted' either way.
Placement depends on the provider and even on the person. A message may reach the inbox at Gmail and land in spam at Outlook. Authentication, sender reputation, the content, the links and past reactions from the recipient all play a part.
The only honest way to learn placement is to test it: send real messages to real mailboxes at the providers your prospects use, and look at where they land. A single test is a snapshot, not a promise. Test before a campaign, repeat after changes to your copy or setup, and treat a drop in replies as a signal to test again.
In Terlido
Terlido's inbox check sends a real test email from one of your connected mailboxes to another connected mailbox, and reports whether it landed in the inbox or in spam. It speaks only for the receiving mailbox's provider.