Hi James,
Based on my experience, this error usually occurs when you are running in Exchange Online mode. Please check it on your side via File > Account Settings and try switching to Exchange Cached mode instead if you are not in to see if the error can be stopped.
If it doesn't help, please verify if the issue persists when copying mails to another folder within a same mailbox.
Any update, feel free to post back.
Regards,
Yuki Sun
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