F-SECURE ANTI-VIRUS FOR MICROSOFT EXCHANGE 8.00 Administrator's Manual page 288

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List of blocked Specify blocked senders. Messages originating
senders from the specified addresses are always treated
as spam.
List of blocked Specify blocked recipients. Messages sent to
recipients the specified addresses are always treated as
spam.
The product checks the sender address from the
SMTP message envelope, not from the
message headers.