F-SECURE ANTI-VIRUS FOR MICROSOFT EXCHANGE 8.00 Administrator's Manual page 283

CHAPTER 8 283
Administering F-Secure Spam Control
The product checks the sender address from the
SMTP message envelope, not from the
message headers.
Max Message Size Specify the maximum size (in kilobytes) of
messages to be scanned for spam. If the size of
the message exceeds the maximum size, the
message is not filtered for spam.
Since all spam messages are relatively small in
size, it is recommended to use the default value.