The qmail-taps-extended patch by Inter7 (extension by Michai Secasiu) provides the ability to save each email which flows through the system. You could decide to log only the messages which have the From or To address set to a local account.
You just have to create a new control file
/var/qmail/control/taps which contains a regex style list of addresses to tap and the email address of where you want the copy sent to.
if you only want to log the messages with the From address @domain.com you would set the configuration like this:
If you want to log only those with the To address @domain.com
if you want the same behavior as the original Inter7's patch: