In order of how often each cause shows up in our customer data:
1. Authentication misalignment. DKIM signs as one domain, From shows another. Some mailbox providers treat the misalignment as spoofing.
2. Domain reputation. You sent to bad addresses, complaints went up, mailbox providers flagged your domain. Recovery requires cleaning your list and rebuilding engagement.
3. Content patterns. Excessive CAPS in subject lines, all-image emails, suspicious URL shorteners, or content that matches known spam fingerprints.
4. Engagement. Mailbox providers learn from user behavior. If your audience never opens, the algorithm learns to route you to spam.
5. Shared IP neighbors. Rare on Inbox OSS because we segment aggressively. Almost never a real cause.
The dashboard's Deliverability tab will surface which of these is hitting you.