Addresses that have been placed in the BCC field are not forwarded. If you have placed a large list of recipients in the To or CC field, all of them will receive the reply. By placing recipients in the BCC field, you can help protect them against receiving unnecessary replies from anyone using the Reply All feature.

Does reply all include BCC?

The answer is no the reply will not be sent to any of the other addresses in the BCC list.

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.