The following warning is received while creating a mailbox in Plesk:
PLESK_WARN: Warning: Unable to apply the Mail Anti-Virus settings because you have reached the maximum number of mailboxes defined by your Premium antivirus license key.
A free version of the Premium Anti-Virus for Servers extension is installed on the server which has limitations.
If the extension is planned to be used further, consider purchasing a license.
If the extension is not planned to be used further, remove the Premium Anti-Virus for Servers extension.