Backup restoration report for the domain example.com has the following warning:
Warning: Restore domain "example.com" Execution of C:\Plesk\bin\mail.exe --update email@example.com -antivirus off -ignore-nonexistent-options failed with return code 1. Stderr is An error occurred while updating mail account: Cannot update mail account: Wrong syntax for command's '-antivirus' parameter. 'off' does not match the pattern '/^out$/'.
Antivirus software is not installed or the feature is not supported by license key.
Purchase and install the license for the antivirus feature: Licensing and Purchase Support
Or simply ignore such warning.