Here's the original-message part of an bounce message: https://i.vgy.me/0KpICY.png

Here's the Gmail web client (Windows, Chrome) display of it:

---------- Forwarded message ---------- From: Chris Jordan <chrisjj@xxx> To: Chris John Jordan <chrisjj@xxx> Cc: Bcc: Date: Sun, 19 Oct 2025 10:32:03 +0100 Subject: Test Test 2

Gmail has fabricated Cc and Bcc fields!

And "Bcc: <blank>" is in this case incorrect, given the real Bcc field has two addresses (which the bounce generator did not include, of course).