Hi,
By 'it remains red' do you mean the email sender and subject show in red in the message list pane? or do you mean the red flag icon will stay after clicking 'Clear Flag'?
Does this issue only happen after sending a reply to that email? What if we try to clear the flag without sending a reply?
Besides, we may also try to start Outlook with /cleanviews switch to reset all custom views to default and then see whether this issue continues. To do this, press Windows key + R to open the Run command, type
outlook /cleanviews and press Enter.
Hope this helps.
Regards,
Steve Fan
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