Using the Read New Mail Button

  Clicking on the Read New Mail button adds new messages to
  the INBOX folder, opens the INBOX folder, opens a default
  Read window (if one is not already open), and displays the
  first unread message in the INBOX folder.

  New mail cannot be added to the INBOX if you are reading
  a message from the INBOX in a Read window other than
  the default Read window or if you are replying to a message
  in the INBOX.

  Additional topics:

  *   Using a Read Window