Software

Only when pane preview is turned on. And the incoming message rests there. Generally if a message has been "seen," it will be marked as "read."

The other possiblity is if you have a shared mailbox. The first user to open or view a message will mark it as unread. So then the answer from the first person is true.

For some e-mail system, you can toggle the status of mail by pressing the insert key to make it read or unread. You might have accidentally press the toggle key (for notes, it is the insert key).

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