I understand the difficulty with translating languages, but.........
"Gmail" is your email service provider and that is Google Corporation.
And the default email app on your phone is also named "Gmail", but they are NOT the same thing.
The first is a Service that allows you to send and receive emails. There is NO requirement that you use the phone's Gmail app to do that.
Your problem as stated was that when you sent multiple emails on the BCC line, that you got an error message.
That just means that one ( or more ) has an invalid address.
That address may be bad now,
or you may have typed in extra characters that corrupted that email address
It don't matter how it happened, you must verify each email address as good, by sending just one email to that person. If you get a reply back, that address is good.
If you are able to send everyone email, and you get replies back from all of them, then you have proven my first point.
You typed in an extra character somewhere and that corrupted that one ( or more ) email addresses.