An HTTP 404 error means the server received the request but could not find the resource at the URL given. In email marketing, a 404 happens when a campaign links to a page that has been moved, renamed or deleted — and every recipient who clicks lands on a dead page instead of the conversion you wanted.
How 404 errors happen in practice for email. A campaign goes out on Monday. On Wednesday, the marketing team renames a product URL or retires a landing page. Every click from that campaign — and from older campaigns still in inboxes — now hits a 404. The click was earned, the open rate was strong, the subject line worked — and the conversion is lost to a routing problem nobody saw coming.
Why 404 errors matter for senders. Broken links destroy trust faster than a bad subject line. Mature email programmes link-check campaigns before send, monitor click destinations in stats afterwards, and prefer redirect-managed URLs that absorb destination changes without breaking the historic email. The work to prevent 404s is small; the cost of one campaign-wide broken link is days of remediation and lost revenue.
Mailpro and Error 404
Catch broken links before they 404 the click
Mailpro link-checks every campaign before send and surfaces broken targets in your stats afterwards — so the click your subject line earned never lands on a 404.