I misremembered - the button to click is "compact now", not "OK". This is why it is important you to post the EXACT language of what you are seeing on the screen. You are using an English language version of THunderbird?
Bug 1607994 Comment 4 Edit History
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I misremembered - the button to click is "compact now", not "OK". This is why it is important for you to post the EXACT language of what you are seeing on the screen. You are using an English language version of THunderbird?