There is a notes.ini parameter RouterKeepRelayedBCCsIntact
which is almost unknown as there is no equivalent in the servers configuration document.
Given that you are using a SMTP relay hosts (as customers of postini or messagelabs do) this setting will allow your outbound SMTP email having several recipients on the BCC list to be sent as a single mail to the relay host instead of sending separate emails for each BCC recipient.
This notes.ini parameter was documented with IBM but to my knowledge there is no documentation from HCL. This can indicate that the notes.ini parameter is obsolete with newer version of Domino, which I have not tested.