Mastadon: Clicking the link in the verifications email goes to a "page cannot be found"

My mastodon instance route66.social is up and running, but users can’t complete the verification process. The email arrives fine, but clicking the link doesn’t work.

Here’s an example from Chrome:

This site can’t be reached email.route66.social’s server IP address could not be found

The email is going through fine, it’s just the user can’t click the verify button, so I have to manually add them.