When you send an email to multiple accounts at once, the system sends a separate email to each one rather than a single email to the whole group.When a single account has multiple emails in it, a single email will be sent to all those addresses at once. So each recipient will see all the other addresses in that account. But separate accounts will not see any addresses in other accounts, because each account is emailed separately.The same is true for CC and BCC recipients—each one is emailed separately, so they do not see anyone else's email address.