How do Gmail and Yahoo handle borderline messages?

Borderline messages that do not clearly qualify as spam or inbox receive nuanced treatment:

Gmail leans heavily on engagement prediction. If similar messages from this sender have been engaged with, borderline messages get inbox benefit of the doubt. User-specific history heavily influences the decision.

Yahoo emphasizes complaint feedback loop data. Senders with low complaint rates get more latitude on borderline content.

Both providers may place borderline messages in inbox initially and learn from the outcome. If users mark it spam, future similar messages get harder treatment.

When the cargo is questionable, the harbor consults your track record with previous shipments.