Enable Anti-Virus Service:
From the myCP -> Email Admin menu, click on the domain name whose email you wish to enable anti-virus service for. On the following page, under the Domain Configuration section, ensure that the check-box next to "Enable Anti-Virus" is checked then save this setting by clicking on [Update Domain Settings] at the bottom of the page.
Disable Anti-Virus Service:
You may disable this service by following the same steps outlined above and merely unchecking the "Enable Anti-Virus" box then clicking on the [Update Domain Settings] button.