How do mailbox providers decide where to place an email?

Mailbox providers run incoming messages through sophisticated classification systems:

Authentication check: Does the message pass SPF, DKIM, and DMARC?

Reputation lookup: What is the sender's IP and domain history?

Engagement prediction: Based on past behavior, will this user want this message?

Content analysis: Does the message contain spam indicators or match known patterns?

User-specific signals: Has this user engaged with this sender before?

These signals feed machine learning models that continuously adapt. The decision is probabilistic, not rule-based.

The harbor inspects your papers, checks your cargo, reviews your history, and predicts whether you are bringing goods or trouble.