using BCC or many messages

Hi

I am just starting to use libesmtp for a project but I am worried about 
effecincy.  
I want to be able to create one message to go to many people but I want 
each to be BCC so they don't know each others email address.   I have 
tried to add Bcc: header but it never get to the Bcc and the to sees the 
bcc.   This like a maillist but each message can be customized. I figure 
95% same 5% custom

how can I create a message and send it to many people and keep the email 
addresses private?

Shaun