Do you have a copy of the raw message? MTAs don't always provide a literal 'Bcc:' header, but often provide 'Envelope-To:' or 'Delivered-To:'.
As a work around. I simple grab emails for each address I need to route mail that come in with BCC's using the following patch.
http://cerb4blog.com/blog/09/07/25/adding-header-info-well-downloading-mail-cerberus-4
Thanks
Robert