def search_BCC(self, query, id, msg):
        """
        Returns C{True} if the message has a BCC address matching the query.

        @type query: A L{list} of L{str}
        @param query: A list whose first element is a BCC L{str}

        @type id: L{int}
        @param id: The sequence number of the message being checked.

        @type msg: Provider of L{imap4.IMessage}
        """
        bcc = msg.getHeaders(False, 'bcc').get('bcc', '')
        return bcc.lower().find(query.pop(0).lower()) != -1

        Returns C{True} if the message has a BCC address matching the query.

        @type query: A L{list} of L{str}
        @param query: A list whose first element is a BCC L{str}

        @type id: L{int}
        @param id: The sequence number of the message being checked.

        @type msg: Provider of L{imap4.IMessage}
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/imap4.py