Some websites offer a service where , when you are sending they could track whenever it has been read. In this case, it won't ask for confirmation(for example you may request a confirmation and if user press yes, you will get message). In this case, just when you open email, it will notify user that you open message but the reciever won't notice anything. There are some modification in message protocol that cause this communication.
I haven't test this with Office365 and Exchange , I am wondering will this work on Microsoft Exchange or do we have mechanism to block this?
I don't want to advertise any company but doing simple search in Bing, you will find providers who are giving you these services.