How do mailbox providers decide inbox vs spam?

MBPs apply multiple filtering layers. First, authentication checks: SPF, DKIM, and DMARC must pass. Failed authentication often means immediate spam or rejection. Properly authenticated mail proceeds to reputation evaluation.

Reputation scoring considers sender history: complaint rates, spam trap hits, engagement patterns, and blocklist status. New senders face scrutiny until they establish positive track records. Known good senders get benefit of doubt; known bad senders get filtered.

Content analysis examines message characteristics: spammy keywords, suspicious links, text-to-image ratio, and structural patterns. Finally, recipient-specific signals matter: individual engagement history with your domain, folder organization habits, and explicit preferences. The combination determines placement.