F-SECURE ANTI-VIRUS FOR MICROSOFT EXCHANGE 7.10 Administrator's Manual page 222

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Number of spam Specify the number of Spam Scanner instances
scanner instances to be created and used for spam analysis. As
one instance of the spam scanner is capable of
processing one mail message at a time, this
setting defines how many messages undergo
the spam analysis simultaneously.
The server must be restarted after this setting
has been changed.
IMPORTANT: Spam analysis is a
processor-intensive operation and each spam
scanner instance takes approximately 25MB of
memory (process fsavsd.exe). Do not increase
the number of instances unless the product is
running on a powerful computer.