Activity processor on emails - doesn't work if sent to BCC recipients?
Neither error or design but by the simple fact that the BCC addresses will never be seen by CiviCRM.

Activity Processor relies upon receiving an email sent to it containing the relevant information.  As you are sending using BCC, the 200 or so email addresses will never appear in the email.

You should send the email from inside CiviCRM and then you will get the result you want.