Creating a backup of incoming mail

This is my master.cf line for dovecot virtual users:

dovecot unix - n n - - pipe flags=DRhu user=vpopmail:vchkpw argv=/usr/local/libexec/dovecot/dovecot-lda -f ${sender} -d ${user}@${nexthop} -m ${extension}

I would like to also write every mail, before delivery to the user, to a backup location like /backup/imap/mm/dd

Possible?

-- 'On whose authority?' demanded Wert. Trymon turned his grey eyes on him. 'Mine. I need no other.' --The Light Fantastic