It appears that the setBccRecipients call on MFMailComposeViewController is not working for Catalyst.
The same code works on iOS, and it works on Catalyst in setCcRecipients and setToRecipients variations correctly, and the documentation says the call is supported, but it returns an error message saying: "setToBccRecipients is not supported" and does nothing.
This is odd, because it is getting the name of the call wrong, as well.
Has anyone else encountered this problem?