I had the same problem with other db files.
After running the examle of IgorG, I got an failed message. I had to delete the complete /var/spool/postfix/plesk folder to be succesfull.
After this I was able to run the mchk utility without any problems.
So to be complete:
# /usr/local/psa/admin/sbin/mailmng --stop-service
# mv /var/spool/postfix/plesk /var/spool/postfix/plesk.backup
# /usr/local/psa/admin/sbin/mchk --with-spam
# /usr/local/psa/admin/sbin/mailmng --start-service
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