Perfect SPF, aligned DKIM and p=reject get a message considered. They do not get it delivered to an inbox, and the gap between those two outcomes is where most “deliverability” work actually lives.
What each side owns
| Authentication decides | Reputation decides |
|---|---|
| Whether the message is from who it claims | Whether that sender is welcome |
| Whether a DMARC policy applies | Whether it reaches the inbox or the spam folder |
| Whether a forged message is rejected | Whether a genuine message is throttled |
| A binary outcome per message | A continuous score per domain and IP, over time |
Authentication is a precondition. A domain that fails it is heavily penalised; a domain that passes it has merely become eligible to be judged on everything else.
What the judgement is on
- Complaint rate. The proportion of recipients pressing the spam button. The single most consequential number, and the only one with a published threshold.
- Invalid addresses. Sending to addresses that do not exist signals a list that was not collected properly.
- Engagement. Opens, replies, and messages moved out of spam. Absence of engagement across a large volume reads as unwanted mail.
- Volume consistency. A domain that sends two hundred messages a day and then four hundred thousand looks compromised, because sometimes it is.
- Spam-trap hits. Addresses that exist only to catch senders using harvested or very old lists.
- Infrastructure hygiene. Reverse DNS, HELO names, TLS support — small signals, each worth little, collectively expected.
Why this belongs in this course
Because the question that follows a completed authentication programme is always “we did all that and mail still goes to spam”, and the honest answer is that those are different problems with different owners. Authentication is a DNS and configuration exercise you can finish. Reputation is a property of how you send, it cannot be finished, and no record you publish will substitute for it.
Nobody can whitelist you
Services offering inbox placement, deliverability guarantees or a relationship with the major providers are selling something that does not exist as a product. Reputation is computed from behaviour. The levers are list quality, complaint rate and sending consistency, and they are all yours.