Reply All
How to change the reply default in 365.
Prevent "Reply All"
If you are sending an email to a large group or an email where all recipients do not need to be replied to, put the email addresses in the BCC field. This prevents any accidental whole group replies.
Change the Default Reply
REPLY ALL is the default reply option in Microsoft 365. If you are not careful, it is easy to accidentally REPLY ALL. Switching the default is easy and will save you from sending a mass email when it is not necessary.