Postfix won't talk to spamd anymore

"spamd: bad protocol: header error: request=smtpd_access_policy"

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:

check_policy_service inet:127.0.0.1:783

The significant part is the "783" - the port number of spamd. You probably want to be connecting to something like amavisd (port 10024) or spampd (note the additional "p").