We have learned that outlook.com has been marking some transfer notifications as malicious today. It appears to be a false positive on outlook.com's end, and we are working with our email provider to get that resolved.
Meanwhile, we have changed the URL of the links in the notification emails, avoiding the URL that outlook.com didn't like. This has resolved the issue for now.
If you use outlook.com for your email, we recommend you check your Control Center "Transfers" page to see if there are any transfers where you may have missed the notification.