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.