Draft permalink cannot be accessed externally

I’m new to wordpress and am using a draft page as a ‘hidden’ page. Selected people receive the permalink via email and can access the hidden page. This works fine except when I check the permalink a day later without first signing into wordpress, no post shows up. If I log in and re-visit the post internally the url (which is exactly the same as the one originally visited) works fine even after logging out.. until it disappears again a day later.

Help!