Related Article: Options to Reply to a TMail
For any TMail, you an post a quick inline response when you do not need to add Writers, add Attachments, or Update/Modify/Delete Sections. When you do a Quick Response, the new Changset only contains a Comment.
This is an inline operation and it does not create a new TMail. (Please see Inline vs Forking Operations for more details.)
In Navigation, select the Message or TMail where you want to post a quickly reply. For example, you can select a Message in Inbox folder, or a TMail in TMail Pins | Recent. (Please see Navigation for more details.)
The TMail details appear in the right side area. The Comments tab is selected by default. If you are already working on the TMail such as in Sections tab, select Comments tab. Enter your response comment in the Click here to respond with a new comment space, as below.
When you start writing your Comment, the Send button appears, as below.
Click on Send button to send the response (add a Changeset with this comment). Click on Cancel button to close the inline response option.