Section
Deliverability
Why a technically correct email still ends up in the spam folder, and what SPF, DKIM and DMARC change about that.
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SPF, DKIM and DMARC
Three bolted-on procedures that together answer a question SMTP never asked: is this sender allowed to do that at all?
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Reputation
Why the question is not "is this message correct" but "do we know this sender and do we like them".
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Bounces and feedback loops
What comes back when something goes wrong — and why most senders are not listening.
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The forwarding problem
A harmless redirection destroys SPF. Why that is not a bug but a consequence.