It's not a direct link. It's a generated indirect link which expires after a certain amount time and looks like "http://e.delta.com/a/xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx" where the x's are generated random characters. This link redirects to the MQM choice page. The email from above from Jan 27th still works, but the indirect links from previous emails are no longer active. I've found that it sometimes doesn't work if I'm not logged into my account, but it works reliably if I'm logged-in first.