When it comes to replying to an email, there are two options available, Reply and Reply all. Let's see what happens if someone replies to a BCC email in Outlook by using these two options.
Reply: If a person uses the Reply option, the email will be sent only to the sender and no other recipients of the email could know about that message.
Reply all: As the name implies, if a person replies to a BCC email by using the Reply all option, the email will be sent to all the recipients of that email including the sender.