Explore the case of the unexpected unsubscribe link

[reefdog]

I sent myself a Hello World test newsletter, and inserted in the bottom of it was an unsubscribe link to http://email.alerts.factstream.co/u/{token}:

Our template, however, doesn't contain any Unsubscribe tags (even though we should add one), and best I can tell, there's no setting w/in Mailgun that's governing this. The link is not being inserted into our real newsletters.

This is a low-priority 🤔 task for figuring out why Mailgun's inserting this and under what conditions it might do it to the real newsletters.

[reefdog]

One possibility: I think we ran the recipient address validation on the real mailing lists and not the dev list. We should test whether sending to unvalidated addresses is what causes Mailgun to force an unsubscribe link.