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A mail server with no PTR record, or one that does not match forward, gets rejected before it says a word. Here is how forward-confirmed reverse DNS works.

Google and Yahoo started enforcing their bulk sender rules in 2024. The requirements are simple to list and easy to fail. Here is what still trips senders up.

One include too many and your SPF record returns permerror, which receivers treat as an SPF failure. Here is how to count your lookups and get back under ten.

Rotating a DKIM key is a publish-first, sign-later dance across two selectors. Get the order wrong and every message in flight fails authentication.

Mutual TLS makes the client prove its identity with a certificate too. Here is the nginx config, the client commands, and the error you get when one is missing.

When a recipient hits the spam button, a feedback loop sends the complaint as a structured ARF report. Here is what is inside one and how to trace it back.