Simple: when one of your To: field of an email it was sent to other that person's identity is exposed so that they're hidden from. The alternative is to send an email to multiple recipients and writing the body of people and that nobody should.
If you are the sender of the email, the answer is yes. You can use a smart email client like Mailbird. Trust me, You can be sure If your email client is listed below, check its specific instructions for how to use the Bcc field to send a message to undisclosed recipients. The problem that arises here is if the original recipient or any Cc'd recipients find out other people were copied on what they assumed was a private email.