I have one site with SBS 2003 where one user often finds that when she logs on to her Outlook client, all her mail in her Inbox is marked as read, even though she has not accessed her mail for a while. E.G. she arrives at the office in the morning, has received 20 emails overnight, and they are all marked as read.
She has changed her password and nobody else knows it.
She accesses her mail via the office PC, her home PC and an Android phone.
The office runs Windows 7 & Office 2007 totally patched up to date. Outlook in the office runs with an Exchange Server account using Cached Exchange Mode.
Outlook on her home PC is Outlook 2010 running Cached Exchange Mode and connecting via RPC over HTTP.
The Android phone POPs the mail and leaves a copy.
The problem has persisted for some months, during which time coincidentally, the office and home office PCs have been replaced and she changed from iPhone to Android.
I have narrowed it down to "something" happening when she atarts Outlook in the office by opening her mailbox via Outlook Web Access and seeing 10 unread messages in the Inbox then getting her to open Outlook and Ta-Da all messages show as read. Refresh the OWA window, & they're all read. There are no rules in her profile.
In summary, the problem has persisted through a complete change of equipment and software on the client side, so I don't beleive it is the configuration of Outlook that is to blame.