The Bcc field is there if you don't want the recipient of an email to see the email addresses of all other recipients who will be receiving the same email. Bcc stands for blank carbon copy. The only recipient address that will be visible to all recipients is the one in the To field. So, to keep maximum anonymity you can put your own address in the To field and use Bcc exclusively to address your recipients.
You will only find the Bcc field in Take Action-Send Email. From the contact log, a separate email will be sent to all recipients, whereas Send Email from Take Action will send the same email to all recipients, allowing contacts to see all other recipients of the email. To prevent this from happening, we have included a Bcc field in Send Email from Take Actions.