Hi @Mark Hudson
Welcome to our forum!
Are you using a new or classic version of Outlook?
How long did the problem occur, and did you do anything when it happened?
Don't all the rules work? As far as I know, in the new Outlook for Mac (as of October 2020), only server rules are available, client rules are no longer supported.
If you're using the classic version of Outlook, please try checking the rule with OWA, deleting and recreating it to verify the results.
Also, please try the repair solutions provided in this document: The rule I created does not work - Microsoft Support.
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