I suspect this could be slightly off-topic

And I suspect there may be faulty reasoning in there somewhere. You fail to specify whether you are trying to extract the Bcc field from messages which have been sent or received. If the former (i.e. you're extracting the headers from mails in the clients "Sent" box), it should work exactly the same way you're already extracting the other headers. If the latter (i.e. the client has received the mail and you're trying to see who else the mail was sent to) I suggest you sit back and reflect on the purpose and implementation of the Bcc field (repeated blows to the head with a blunt object may aid in this reflection).