How to Reply to a Message in Outlook 2013
To reply to the people in the From field, click the Reply button. To reply to the people in the Cc field and the From field, click the Reply All button.
The Reply screen appears.
You may get (or send) e-mail that's addressed to a whole bunch of people all at one time. Ideally, at least one person should be named in the To field; more than one person can also be in the Cc field, which is for people to whom you're sending only a copy.
Little difference exists between what happens to mail that's going to people in the To field and mail that's going to the people in the Cc field — all of them can reply to, forward, or ignore the message. You don't always need to reply to the people in the Cc field, or you may want to reply to only some of them.
If you do, click the Reply button and add them again to the Cc field. Or, you could click the Reply All button and delete the users from the Cc field who you don't want to include.