How to enable antivirus server-wide?
Only Kaspersky Antivirus can be enabled on a server-wide level in Tools and Settings > Antivirus.
What about Premiun Antivirus (Dr.Web), it can be switched on in Plesk GUI only on a per-mailbox basis and only after mailboxes are set up. For more details, refer to Plesk Administrator Guide.
To enable Premiun Antivirus for a certain mailbox in Plesk GUI, go to:
Domains > example.com > Email Addresses > email@example.com > Antivirus > Switch on antivirus protection for this email address
It is possible to enable antivirus for all mailboxes on a server or on a certain domain using command line. Visit the following article for more details:
128903 [How to] Enable spam filter and antivirus for all mailboxes
Note that both antiviruses require an additional license key with annual renewal.