I don’t know how to describe this one other than to say the terminology used on the block doesn’t match typical email expectations. In Gmail (or pick your email client) if I put a group of email addresses into the address field, everyone can see who else is on the email. If I put someone on BCC, they see they’ve received an email as a BCC and can see everyone on the email, but the primary recipients can’t see the BCC.
That doesn’t appear to be how the email block works in Backendless. As far as I can tell, all of the input fields for recipients do the exact same thing.
Test email block with BCC
Results look like I was the primary recipient of the email
This is what a Gmail BCC looks like
Sending an email to multiple recipients
Results look like I was the only one on the email, and look just like when I was BCCed from Backendless