To enable AntiVirus for all accounts in Plesk you can use the following script:
#!/bin/bash mysql -uadmin -p`cat /etc/psa/.psa.shadow` psa<<-EOT UPDATE mail SET virusfilter = 'any' WHERE postbox = 'true' AND virusfilter != 'any' EOT
You can run it daily from the cron to keep your mailboxes protected.