The GMail API has many extra functions that can be usefull. For example IsUnread tells if a message was read.
We use the Cloud Pack to read status mails and would be convenient if we can select only new and unread messages.
See Google API at:
var thread = GmailApp.getInboxThreads(0,1)[0]; // Get first thread in inbox
var message = thread.getMessages()[0]; // Get first message
Logger.log("is unread? " + message.isUnread());