Different URL in outbound email

All of the URLs in outbound email (notifications, account conrfirmations etc) are using a different subdomain to the forum - where the URL should show talk.domain.com, it’s showing up as link.domain.com. I’ve obviously set this someplace, but it’s not in app.yml nor anywhere in the Discourse config that I can find.

Where would this be set?

Your outgoing mail provider most likely has some kind of click tracking, turn that off.