Sender reputation

Every message you send is a small vote. Opened and replied to, the vote is positive. Deleted unread, marked as spam, or bounced, the vote is negative. Providers keep a running tally per domain and per mailbox, and that tally is what decides placement. The words in the message matter less than the history of the sender.

A new mailbox has no history, so it has no reputation. That is why warmup exists: it builds a history of positive votes before the first real send. It is also why volume matters. A mailbox that goes from nothing to hundreds a day looks like a spammer, because that is what spammers do.

In outbound engineering, reputation is read rather than assumed. Each mailbox gets a daily reading against real inboxes, and the reading sets how much it may send that day. A healthy mailbox settles around 60 messages a day. When the reading dips, the cap comes down before the damage is done.