Gmail

Make "Reply all" your default setting

1. On your computer, go to Gmail.
2. In the top right, click Settings .
3. Click Settings.
4. In the "Default reply behavior" section, select Reply all.

You can still choose to reply to just one person, but "Reply to all" will be the first option.

Outlook

A method that works for both Office 365 and Exchange 2016 On-Premises is to go via the Options screen.

1. Click on the Gear icon in the top-right corner left from your user picture.
2. From the menu that open choose:Exchange 2016 On-PremisesOptionsOffice 365section: My app settings-> Mail
3. In the left pane choose: Mail-> Automatic processing-> Reply settings
4. Set your default response to: Reply.
5. Press the Save button at the top.
6. Click on “Options” in the top left corner to return to your mailbox.

Setting the default Reply action via the Options screen.