Novell GroupWise 2012 Support Pack 3
After the installation of the GroupWise co-existence free busy service, users in Outlook OWA could no longer delete
un-opened messages in OWA.
Start IIS Manager, expand Sites, Default Web Site, EWS. Then in center dialog box, double click on IIS, Authentication, and make sure "Basic Authentication" is DISABLED. If it is Enabled, you have the problem with not being able to delete mail messages.
This was an Exchange EWS configuration issue.