As you know, recipients can't tell who you included in the BCC field, or even if you used the BCC field at all. But that doesn't mean you can't. To see who you BCC'd in a previous email, just open the Sent mail folder and open the message. You'll see the BCC field preserved for future reference.

Is BCC really hidden?

BCC stands for “blind carbon copy.” Unlike with CC, no one but the sender can see the list of BCC recipients. ... However, the BCC list is secret—no one can see this list except the sender. If a person is on the BCC list, they'll see only their own email on the BCC list.

Does BCC show the recipient?

If you add a recipient's name to the Bcc (blind carbon copy) box in an email message, a copy of the message is sent to the recipient that you specify. Any recipients added to the Bcc box will not be shown to any other recipients who receive the message.