Given admin user can see iNotes interface and other users end up with WebMail (older) interface, I’d wonder two things:
The mail templates non-admin iNotes users are using. mail8.ntf? dwa7.ntf? or mail7.ntf? If just the latest iNotes UI is desired for the users - Upgrading those users to mail8.ntf (it should be mail85.ntf in case 8.5.x release is in use) may solve the problem
What value non-admin users’ mail DBs have in their icon notes. One is $FormsTemplateFile item and another is $WebHybridDb item. The former should point to the correct path of Forms8.nsf. The latter should be “1”. Both can be seen in NotesPeek (http://www-01.ibm.com/support/docview.wss?uid=swg24005686).