i know i'm reviving an old topic but i'm reviving it because the advice is wrong
You can;'t delete a message after you've sent it. Not even if you go to your "Sent" section and delete it, the reason that it says that you still have a new message is simple.
Your on a page. You open your mailbox and read it. then you close your mail box and your still on the same page which still says that you have one unread message. So all you have to do is refresh your page and then it'll recheck you mailbox and tell you that you have no new messages. You could try it if you don't believe me