Anyone have experience with recipient_bcc_maps in postfix?

 I got "always_bcc" to work easily, but must be misunderstanding 
"recipient_bcc_maps"

I added "recipient_bcc_maps = hash:/etc/postfix/recipient_bcc" to 
main.cf, and created "recipient_bcc", and ran postmap against it. The 
content of recipient_bcc is "user at domain.com acrchive at domain.com".

It's not working,. so obviously I am misunderstanding how it works. 
Ultimately, I need to archive mail for 20 different domains, each 
needing their own archive folder.