Using older versions of Outlook for Mac in business environments
A common cause of these rule errors is a bad email address in one of the rules (possibly a typo) or an invalid folder name (possibly deleted or renamed?). Please check the most recent rule(s) you created and edited to see if all parameters are currently valid. You can also narrow down the specific rule by selectively removing/re-adding individual rules. Unfortunately, all rules are sent to the server in one transaction, and Outlook currently does not have code to identify and display the specific rule that failed. We've logged that as a future improvement.