For a TMail, you can add a Read Acknowledgement and send it to all associated Writers to indicate that you have read a particular Comment or List of Comments.
In Navigation, select the Message or TMail Pin corresponding to the TMail for which you want to add a Read Acknowledgement. For example, you can select a Message in Inbox folder, or a TMail Pin in TMail Pins | Recent. (Please see Navigation for more details.)
In the right hand side, locate the specific Comment for which you want to add a Read Acknowledgment. Click on the small down arrow for this Comment, as below.
Select Add Read Ack option. TMail21 shows you the details of Read Acknowledgement that you have read Comments from #1 to #n where n is the selected Changeset, as below.
Click on Acknowledge I have read. A changeset is added to the TMail with your acknowledgement. A Read Acknowledgement icon is also present.