When a mail is sent to you, the sender can enable Return receipt in the delivery option which will help him/her know that you have read his/her mail. There is no direct means to avoid sending the confirmation message to the sender when you open the mail. Incase you want to avoid triggering the automated reply to the sender but want to have the message read, the way out is to use a simple trick. Just select the mail without opening it, click Reply tab on the action bar and choose Reply with History. A new memo opens as a thread showing the contents of the mail received by you earlier. You can read the message here and can also open any attachment without the sender knowing that you have read the message.