When you reply normally, the reply goes to A. The email contains no information about the other 16 people, or even that it was BCCed to 16 people. All you get in your email is that it was from A to B and you were BCCed. So imagine A sends an email to B and also BCCs 17 others including yourself. It simply can’t reply to them, only to the original reply-to email address (usually the sender) and maybe to other “to” and CC email addresses if you select “reply to all.” But it cannot know about any BCCed people so it can’t reply to them. So when you reply, there is no way that your email app can even know their email addresses, or that it was even sent to those people.
If it did, then it would be easy to view the source of the email and see who else got it, defeating the purpose of BCC.
When you get an email that has been BCCed to others, the email does not contain the information about those BCCed people.
