Email can fail to reach your inbox for a handful of common reasons, such as a full mailbox, a forwarding rule, an inbox filter, or the sender using the wrong address. These are issues you can usually identify and fix yourself in just a few minutes.
A growing list, a new product launch, or a shift to more frequent email campaigns all create real pressure to send more email, faster.
SPF passes. DKIM passes. The spam checker gives a clean report. No blacklist hits. By every available test, the email looks ready to send, and by every available test, it should land in the inbox.
You paste an email header into an analyzer and everything looks right. SPF passes. DKIM passes. DMARC passes. The Received chain looks normal, and there are no obvious routing or authentication issues. Yet the email still lands in Spam.
A campaign goes through a spam checker before it ships. The tool comes back with a 9 out of 10, a green "low risk" label, maybe a checklist of passed items.
An "SPF failed for IP" message, whether it shows up in a bounce notification, a DMARC aggregate report, or a mail server log, points to a specific and usually fixable problem: a sending IP address wasn't found among the sources authorized in the domain's SPF record.
DKIM is often described as one of the more reliable authentication protocols, and in a technical sense it is: the cryptographic verification either matches or it doesn't.
A message can pass SPF. It can pass DKIM. And it can still fail DMARC. This surprises a lot of people setting up DNS authentication for the first time, because it seems to contradict how the three protocols are usually described together.
SPF records look simple enough to write from memory, and that's exactly what causes most of the problems with them.
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