'View in browser' link on mailing gives 'Access denied' error
You need to change the permissions to allow the public to view the mailing.

This permission is called: 'CiviMail: view public CiviMail content'.  This should be assigned to the anonymous role (or all user roles).

You also need to ensure that the mailing visibility is set to 'Public pages' for each mailing that you want to be available to view.  This setting is found under 'Publication'.