If you use the POP protocol, AND you left the computer ON, AND outlook/outlook express is set to periodicaly get new messages automaticaly, you should disable that feature, or, at least, don't let outlook/outlook express active when you are not actively using it.
In this case, your e-mail client did fetch the messages before you read or suppressed them. It have NO way to detect that you read them later and change the read status accordingly. It also can't tell if any message got deleted after it got downloaded.
If you use the IMAP protoco, this should not happen as the messages list, including the read status, is updated in real time.
That mean that if you access your messages from two devices at the same time and you read a message on one device, it will be updated as read on the second device and if you delete a message, it will also disapears from the second device.
Electro, March 2014
