How can broken CNAME or tracking domains affect deliverability?

Your email goes out perfect, but when someone clicks the link, they get an SSL error or a dead page. That's not just a user experience problem. It's also tanking your sender reputation.

Here's the chain reaction: You've set up a custom tracking domain using a CNAME record. That CNAME points to your ESP's servers so click and open tracking works smoothly. But if that CNAME is misconfigured or the SSL certificate expired, the whole thing breaks.

When a link breaks in an email, ISPs flag it as suspicious. Spam filters notice broken links. Recipients see error pages and hit the back button, sometimes they even report the email as phishing. Your ESP might fall back to a shared tracking domain to keep things running, but shared domains have mixed reputation. Thousands of senders use them, so if one person's abusing the domain, everybody's reputation suffers.

And the first sign of trouble is usually user complaints. Recipients click a link, get an SSL certificate error or 404, and either assume it's phishing or give up. You might also notice that click tracking numbers suddenly drop or disappear entirely.

To diagnose it, test your links manually. Click through an email you just sent and see what happens. Does the link resolve? Does the page load cleanly? Or do you hit an error? Then log into your ESP and verify that your custom tracking domain is configured. Check that your CNAME record is pointing to the right place. Finally, verify that your SSL certificate is valid and not expired.

MXToolbox can help validate your CNAME record resolves correctly. If it's broken, your DNS is pointing to the wrong place or the record doesn't exist.

Fixing it depends on what's broken. If your CNAME points to the wrong server, update the DNS record in your domain registrar. If your SSL certificate expired, renew it (most ESPs handle this automatically, but check). If open or click tracking isn't firing, resync your tracking domain in your ESP settings.

Don't let this slide. Broken tracking domains affect every email you send. Do a quick link test today. If anything fails, fix it before your next send.