Hi,
What's the type of your email account? POP, IMAP or Exchange? What's the version of your Outlook client? Please try exiting Outlook desktop client and see if the issue can be reproduced on the web mail.
In this case, it is suggested to check if you have set any rule which may lead to this issue. If there are multiple rules or you have difficulty finding the culprit one, please turn off all rules via File > Rules & Alerts and see if the problem
still exists.
Besides, there could be some add-ins in your Outlook client that cause this issue, so it is suggested to start your Outlook in safe mode (Press Win + R, type “outlook /safe”, press Enter.) and see if the issue continues.
Hope this can be helpful.
Regards,
Aidan Wang
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