{"id":4024,"date":"2025-03-05T12:52:17","date_gmt":"2025-03-05T12:52:17","guid":{"rendered":"https:\/\/www.warmy.io\/blog\/smtp-error-451-4-7-24-dmarc-policy-failure-causes-solutions\/"},"modified":"2026-04-05T14:11:24","modified_gmt":"2026-04-05T14:11:24","slug":"smtp-error-451-4-7-24-dmarc-policy-failure-causes-solutions","status":"publish","type":"post","link":"https:\/\/www.warmy.io\/blog\/smtp-error-451-4-7-24-dmarc-policy-failure-causes-solutions\/","title":{"rendered":"SMTP Error 451 4.7.24: Email Blocked Due to DMARC Policy Failure \u2014 Causes and Fixes"},"content":{"rendered":"

Imagine this; you have already put in hours working on the perfect email campaign, you personalized every message, you took the time to double-check everything, there are no typos and you hit \u201csend\u201d only to find out your emails are getting blocked. No warning, no real explanation\u2014just an obscure bounce message: \u201cSMTP Error 451 4.7.24 \u2013 DMARC\u2002Policy Failure.\u201d<\/span><\/p>\n

If this is starting to sound familiar, you\u2019re not alone. A lot of businesses and email marketers struggle with this annoying error without being able to fully comprehend why their emails are not accepted. In this article, we\u2019ll explain what SMTP 451 4.7.24 means, why DMARC failures cause email rejection, as well as how to resolve them so your emails find their way into inboxes instead of spam folders.<\/span><\/p>\n

What is SMTP Error 451?<\/b><\/h2>\n

SMTP (Simple Mail Transfer Protocol)<\/span><\/a> is a\u2002set of rules that servers follow to send and receive messages. But when things go wrong\u2014especially with security authentication\u2014your email server\u2002may respond with an <\/span>SMTP 451 error<\/span><\/a> indicating a temporary rejection. The 4.7.24 error code is a special case\u2002that indicates a DMARC (Domain-based Message Authentication, Reporting, and Conformance) policy failure, which means your email failed an authentication check.<\/span><\/p>\n

What is DMARC and why is it important?<\/b><\/h2>\n

Now more than ever, email security is vital. Cybercriminals leverage email\u2002vulnerabilities to impersonate businesses, send phishing emails and commit fraud daily. This is where DMARC steps in.<\/span><\/p>\n

It isn\u2019t just another email security protocol. <\/span>It\u2019s a policy layer that ensures only legitimate emails sent from your domain reach inboxes.<\/b> attackers can send emails that appear to be coming from your\u2002address (spoofing it) when they\u2019re really phishing attempts to steal sensitive information from your customers or employees.<\/span><\/p>\n

For companies that leverage email for marketing, sales, and customer communication, improperly configured DMARC policies can lead to rejection of their email and erode\u2002inbox placement rates, resulting in poor deliverability and lost revenue.<\/span><\/p>\n

How DMARC works<\/h3>\n

DMARC works by telling receiving mail servers how to handle emails that fail authentication. This is done through SPF and DKIM alignment checks in order to deliver, quarantine, or reject an email.<\/span><\/p>\n

In order to understand DMARC\u2019s effectiveness, we first need to cover the two\u2002main email authentication technologies DMARC builds upon: SPF (Sender Policy Framework) and DKIM (DomainKeys Identified Mail).<\/span><\/p>\n

Overview of SPF (Sender Policy Framework)<\/h3>\n

SPF is like a guest list for your email domain. It tells receiving mail servers which email servers are authorized to send emails on behalf of your domain.<\/span><\/p>\n