There are two ways to reply to a message:
click on the BLUE reply button at the bottom of a thread. This simply adds your message to the thread (the way it worked on the old software).
click on the GREY reply button at the bottom of each message. This also adds your text as above, but
-in addition- it
a) sends your message text as an ordinary email to the author of the message. For example, if I reply to EOD this way, EOD will receive an email from JPeelen.
b) Also, the current thread subject is added as permanent 2 lines at the top of the recipients (EOD’s in this example) logout menu.
My proposal is to use the BLUE reply button, unless in special circumstances.