Hi @Duncan Allcoat ,
From what I've searched, it may be related to client rules, it is recommended that you try clearing client rules to see if the problem will be solved: Press the Windows + R keys on your keyboard to bring up the Run utility. Type in outlook.exe /cleanrules and press.
Besides, to eliminate the impact of profile corruption, it is recommended that you create a new Outlook Profile:
- Open Control Panel and select Mail.
- Click on Show Profiles and then Add to create a new profile.
- Set the new profile as the default and see if the issue persists.
If the issue continues, you can try the Support and Recovery Assistant: Microsoft offers a tool called the Support and Recovery Assistant for Office 365, which can help diagnose and fix issues with Outlook. For more details, please follow the link below: https://www.microsoft.com/en-us/download/details.aspx?id=103391&msockid=05010d57d4016b3b266a18c7d0016d19
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