How does inbox placement differ across mailbox providers?

Each major mailbox provider uses different signals and algorithms for placement decisions:

Gmail weighs user engagement history heavily. It learns from opens, clicks, and how users interact with similar senders. Machine learning adapts continuously.

Microsoft (Outlook, Hotmail) uses SmartScreen filtering and emphasizes authentication, IP reputation, and complaint rates.

Yahoo/AOL focuses heavily on complaint feedback loops and sender reputation scores.

Smaller providers may rely more on third-party blocklists or basic content filtering.

The same email can land in inbox at one provider and spam at another. Each harbor has its own port authority with different inspection criteria.