How can broken CNAMEs cause reputation issues?

You set up a custom tracking domain (a CNAME pointing to your email service provider's tracking servers), but somewhere along the way it broke. Now what happens to your reputation?

First, recipients hit broken links. They click on what they think is a real link, and it doesn't work. They get frustrated. Some mark your email as spam out of annoyance. Others assume you're phishing (a broken security certificate doesn't help). Each spam complaint tanks your sender reputation score at that mailbox provider.

Your ESP might fall back to shared tracking. When your custom CNAME is broken, some email service providers automatically route tracking through a shared domain owned by the provider. Now your opens and clicks are tracked on shared infrastructure with thousands of other senders. If any of those senders are spammers, you inherit their reputation problems. You lose the reputation benefit of your own domain and get dragged down by strangers.

You lose visibility into engagement. Without working tracking, you can't see opens or clicks. You're flying blind. Bounce rates, complaint rates, and engagement metrics disappear from your dashboard. By the time you notice delivery is tanking, the damage is already done.

The fix is straightforward but critical. Test your CNAME before launching campaigns. Make sure the domain resolves correctly, the SSL certificate is valid, and tracking pixels actually fire. If you're not sure how to check, ask your ESP's support team to validate it for you. A broken CNAME is such an easy thing to miss, but the reputation hit is real. Understand CNAME basics and how sender reputation works. Then make DNS validation part of your routine deliverability audit.