To forward outgoing mails of particular user to another account using sender_bbc_maps in Postfix:
To main.cf add the following entry:
sender_bcc_maps = hash:/etc/postfix/sender_bcc
and then create a file in /etc/postfix called: sender_bcc and then add the following entry to sender_bcc:
user1@domain.com anotheruser@domain.com
And then type the following command:
postmap /etc/postfix/sender_bcc
and restart Postfix. Now on, emails sent from user1@domain.com will be a blind carbon copy to anotheruser@domian.com