Some users are unable to delete emails in their 2010 mailbox using OWA when proxied through Exchange 2013. The error is:
An unexpected error occurred and your request could not be handled. Access is Denied. Check credentials and try again.
This error only occurs for some users and only when proxied by Exchange 2013. If the user logs on directly to OWA on an individual 2010 CAS server in the load balanced array, they have no difficulty in deleting emails. Oddly enough, when being proxied, the user can double click to open an email and then delete it successfully.
Have been all over the web and forums and cannot find any hint of a solution.