I just sent an email with all the email addresses in the BCC field, which should have hidden all the email addresses from recipients. But I received an email from one of the recipients saying that they could see the whole email list. I checked my sent mail and indeed all the email addresses were in the BCC field. This is really troubling as I often need to send BCC emails. Why would this have happened? And how can I prevent it from happening again? Thanks so much.