What is the purpose of the numeral to the right of Sent in Apple Mail? I realize this is not specific to Apple Mail, but recently someone asked me this and I had not a clue of its meaning. I also am having no luck searching Google or Bing for the answer. The closest thing I have found to an answer was "refresh the page until it goes away". 

So if the numbers on the majority of labels means "unread" and the one in outbox mean "unsent", what does the number next to sent mean?
It means "unread".

Now that does not seem to make much sense, and I think it doesn't, but that is what it is. You can select a mail from your sent folder and mark it as unread, and the count will increase.