i work at an office where all of anyones emails arrive at every Mac at once. Now the problem is, if, for instance, our secretary reads one of our emails, the Mail is automatically being marked as read on all the other Macs. Also if she sends an Email to another employee and reads it in her outbox, the email is being marked as read on all other pcs (hackintoshs) on our network as well. This wasnt the case prior to upgrading to Mavericks.
Im pretty sure this is just some kind of setting, but i havent found it yet...
Btw we're on 10.9.1