Bcc not truly hidden by default in Mail app

I composed an email requesting bids from five contractors. The first was in the primary email box and the other 4 were entered in the bcc: field. What I discovered is that by default, everyone on the email can see everyone else. Not until I changed the addressing option did it become a blind cc.

[Re-Titled by Moderator]

Original Title: FYI, bcc isn’t by default