Outlook additional mailbox problems with categories and unread message total

Hi,

I have a user who is experiencing problems with Outlook 2010 on Windows 7 Professional 32bit. The user has access to an additional mailbox. When accessing this mailbox the number of unread messages always appears as one less than all other users who have access to this box. Also each time the user marks an email with a category after 23 seconds the category is automatically removed from the email. Any ideas?

Thanks

Oliver Adams