This behaviour occurs if the following option is set to 'Yes' in Domino:
If each recipient's address does not appear in any address header, then add their address to the BCC list
For details, please see http://publib.boulder.ibm.com/infocenter/domhelp/v8r0/index.jsp?topic=%2Fcom.ibm.help.domino.admin.doc%2FDOC%2FH_SETTING_ADVANCED_INBOUND_MIME_OPTIONS_OVER.html.