Why does Exchange show other users logged into my Mailbox?
Have you noticed that when you use System Manager in Exchange Server 2003 to browse Mailboxes that the “Last logged on by” column frequently displays incorrect account information? For example, you may have noticed that your mailbox account shows other people listed in the “Last logged on by” column. This is often very confusing for the administrators. They are concerned about the security and finding several people being logged in to other people’s mailboxes is not very comforting. Rebooting the server won’t make a difference and even if your server is up to date with all the patches and hotfixes, the behavior still remains the same.
The reason for this “feature” displaying other users as “Last logged on by” in Exchange is that the other users are querying free/busy time. You can test this easily with a meeting entry and query someone else’s mailbox for free/busy time. You will notice that your name will show up in the “Last logged on by” column.