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This error, possibly showing up in /var/log/mail.info, indicates you have postfix attempting to communicate directly with spamd. Which doesn't work. "request=smtpd_access_policy" is part of the postfix protocol, which spamd does not speak. The only thing that can communicate directly with spamd is spamc.
You probably have a line in your /etc/postfix/main.cf similar to:
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The significant part is the "783" - the port number of spamd. You probably want to be connecting to something like amavisd (port 10024) or spam*p*d spampd (note the additional "p").