Modern mailbox filtering weighs how recipients actually treat a sender's mail, not just authentication and content: opens, clicks, replies, moving mail out of spam, or deleting without opening are all signals, aggregated per sender and increasingly per individual recipient relationship.
This is why sending to highly engaged recipients first during warm-up and prioritizing engaged segments in ongoing sends both matter: engagement compounds, and a sender with consistently positive engagement gets more benefit of the doubt from filters.
It also means a technically perfect, fully authenticated send can still land in spam if the recipient list has poor engagement history, since content and authentication are only part of the filtering decision.