If you want the mail headers to be updated with the latest version of ClamAV that's installed, I found i needed to run
/var/qmail/bin/simscanmk -g and restart qmail (qmailctl restart)
This will update /var/qmail/control/simcontrol.cdb and the email header with the installed version e.g.
Received: by simscan 1.1.0 ppid: 25356, pid: 25358, t: 2.0586s
scanners: clamav: 0.88/m:36/d:1358 spam: 3.1.0
Regards,
