Hello paulblackburn,
I need more input, because I can't reproduce this error.
Tested: Newsletter sent and published to a mailing list with restricted access to registered users. If I login at front end as SuperUser I can see the newsletter in the list and a click to this newsletter opens the newsletter without problem. Do You perhaps use a campaign?
By the way: You should also be more precise with the specification of the used versions, if you want help. Latest is for BwPostman in some days not the latest, because a new release is planned for Saturday. So if someone reads this thread at Sunday, Your current version is not the latest.
I assume, with 'mail box' You denote BwPostman?
Hth
Romana
