I just tested this again here on meta and here we don’t have that Unsubscribe link at the top, like in your screenshot. So it likely does have to do with your email service which is sending your emails as though they are newsletters, not transactional email. You’ll have to ask them about it.
What happens when you click on that Unsubscribe link? It won’t take you to Discourse because there is no unsubscribe function for transactional emails like signup confirmation.
