[Exim] Save to maildir in system filter

Morning all,

I'm attempting to write a system filter which would save all messages
scoring between 4 and 5 with SpamAssassin to a maildir folder, independent
of any user account on the system. I'm using Exiscan-ACL, and have the
spam_score_int variable available for the checking. What I'm planning on
doing is saving these messages using the following command...

unseen save /var/spool/spamassassin/possiblespam/

What I'm wondering is if I have to set the system_filter_directory_transport
for this to work? The only maildir transport I have is the default
maildir_home one, however, I'm not saving these messages to a user's
account. In fact, I don't have any local users, this is a gateway machine.
Can this work?

PS - For the curious, I'm planning on scanning that folder for spam that
just fell under the radar, and training Bayes with it.

Thanks,
Steven
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Steven Dickenson <sdickenson@???>
Network Administrator
The Key School, Annapolis Maryland