We apologize for the instability of composr.app and appreciate your patience. We are working on the statistics addon and trying to find an optimal way to store and render data. Unfortunately, we have yet to find a solution that can handle the traffic (and therefore, tens of millions of statistical records) of composr.app. We're working hard on one.
According to their docs:
"You can include cc addresses when sending through SMTP. Whether recipients see each other's email address(es) depends on your account settings and whether you've set the X-MC-PreserveRecipients header."
It makes no mention of BCC, but does discuss the above "alternative" to BCC that they seem to be handling (i.e. turning CC into BCC via an alternative setting or header). So I suspect they may simply not have BCC implemented.
I wonder if disabling "Enable Blind CC (BCC)" makes a difference, maybe Mandrill doesn't like BCC.
"You can include cc addresses when sending through SMTP. Whether recipients see each other's email address(es) depends on your account settings and whether you've set the X-MC-PreserveRecipients header."
It makes no mention of BCC, but does discuss the above "alternative" to BCC that they seem to be handling (i.e. turning CC into BCC via an alternative setting or header). So I suspect they may simply not have BCC implemented.
They also have an option to forward all email to another address, so I have turned the Composr one off and gone for that.