vi MailScanner.conf
Archive Mail = %rules-dir%/archive.rules

vi archive.rules
FromOrTo: a@sapow.idv.tw yes forward b@sapow.idv.tw

always_bcc (default: empty)

Optional address that receives a "blind carbon copy" of each message that is received by the Postfix mail system.

Note: if mail to the BCC address bounces it will be returned to the sender.

Note: automatic BCC recipients are produced only for new mail. To avoid mailer loops, automatic BCC recipients are not generated for mail that Postfix forwards internally, nor for mail that Postfix generates itself.

也就是說
在你的 main.cf 中加入一行
always_bcc=username@extmail.com.tw
就可以了

always_bcc會把所有信件備份下來,如果只要備份特定人的信件的話sender_bcc_maps和recipient_bcc_maps應該比較適用吧....