What’s the difference between link CNAME and email sending domain?

Your subscribers see your sending domain every time they look at the From address. They probably never notice the link tracking domain. But spam filters notice both, and that's where things get interesting.

Your sending domain is the domain in your From address. If you send from captain@deepcurrent.io, then deepcurrent.io is your sending domain. It needs SPF, DKIM, and DMARC records in place so receiving mail servers can verify you actually sent the email. Your sending domain also carries your sender reputation. A long history of good engagement helps it. High complaints or big bounce rates hurt it. This is the domain that affects whether you land in the inbox or the spam folder.

Your link tracking domain (also called a link CNAME) is a different domain or subdomain that wraps the URLs inside your email. When someone clicks a link in your campaign, the click goes through that tracking domain first, your ESP logs the click, and then the reader lands on the actual URL. A branded tracking domain looks something like clicks.deepcurrent.io instead of the generic tracking URL your ESP would use by default. You set it up by adding a CNAME record in your DNS that points to your ESP's tracking servers.

These two domains do different jobs. Your sending domain is about who sent the message. Your link tracking domain is about the content inside it. Spam filters check both, but they check them differently. The sending domain goes through authentication checks (SPF, DKIM, DMARC). The link tracking domain goes through reputation checks on the URLs themselves. URL reputation databases flag domains that appear in phishing emails or spam campaigns, and if your tracking domain is shared with spammers, those links can trigger filters even when your sending domain is perfectly clean.

Here's where things go wrong in practice. If your CNAME isn't configured, your links may fall back to your ESP's default shared tracking domain. That domain is used by thousands of other senders. Some of them aren't careful. A shared tracking domain that picks up a bad reputation drags your click-through links down with it, even though your sending setup is solid. It's a bit like shipping a quality product in someone else's packaging that's already on a watchlist.

The other common failure is an SSL certificate issue. If your link CNAME is set up but the SSL cert isn't provisioned correctly, recipients see a security warning when they click your links. That's a deliverability problem and a trust problem at the same time. Many readers won't click through, and some mail clients will flag the message entirely.

If you're unsure whether your CNAME is verified and your SSL is working, the next question covers what breaks when a CNAME isn't verified and how to spot the signs. You can also check your overall sending setup with our free Email Header Analyzer to see what your authentication actually looks like in practice.