If you have click tracking enabled, your links get rewritten to point through a tracking proxy — something like track.buttondown.email — so we can record when a subscriber clicks. (You can read more about how all of this works in our post on insourcing analytics.)
We've now added the ability to use your own custom sending domain as the click tracking proxy instead of our shared one. In our testing, this improves both deliverability and the general trustworthiness of links in your emails: subscribers see your domain in the link, not ours.
If you're using our managed domain setup, this is already rolling out for you automatically. If you're using a manual DNS configuration, head to your sending domain settings to add the additional CNAME record.