Hi @Zoey Liang ,
Microsoft Q&A currently only supports English, could you please edit your question into English? Then we can help to solve your problem, thank you for your understanding.
According to my understanding, do you mean a user has left the company and you want to prevent the user from being added to the BCC list?
Is the member's mailbox still preserved or has it been deleted or disabled? Under normal circumstances, when a mailbox is not existent, even if it is added to the recipient list, it will show that the email address is invalid and the mail to that user cannot be delivered. So it’s not very clear about your situation, can you elaborate?
In addition, if the mailbox has been removed that your issue is that the user is included in a group that in the BCC list, you can delete the member from the group in the EAC as an administrator.
Another way is that you can create a mail flow rule to delete mails sent to this user, the conditions are as follows:
Regards
Shaofan
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