What does it mean for an email message to be “old”?

Question

I just recently converted my mbox over to Maildir, and when I opened up mutt, I saw an 'O' flag on some messages. Looking this up, I find that it means the message is marked as "Old". However as soon as I read the message, the flag disappears.

This seemed rather strange to me. What does it mean for a message to be "Old", and why would it no longer be old if I have read it?

Answer 1

An old message is an unread message that is not new. A message is new if this is the first time the mail reader has noticed it in your mailbox. So if you start your mail reader and there are five new messages and you quit without reading any of them, the next time you start the reader there should be five old messages.