The most common fix is to turn off Outlook’s Conversation Clean Up option that causes the original message to be treated as changed, then restart Outlook. Microsoft support also recommends keeping Outlook updated and restarting your machine, since the issue can be tied to known Outlook/Office bugs.

What to change

  1. Open Outlook.
  2. Go to File > Options > Mail.
  3. Find Conversation Clean Up.
  4. Uncheck “When a reply modifies a message, don’t move the original.”
  5. Click OK , close Outlook, and reopen it.

If it still happens

  • Make sure Outlook is fully updated: File > Office Account > Update Options > Update Now.
  • Restart Windows after updating.
  • If you use an IMAP account, some reports say the issue can also be related to how Outlook marks items as read in the reading pane or to folder/root settings.
  • A quick workaround is to click a different email first, then try moving the problem message again.

Why this happens

This warning usually appears when Outlook thinks the message changed after it was opened, read, replied to, or auto-saved, so the move action gets blocked. Microsoft community reports also describe it as a long-running Outlook issue, especially with some IMAP setups.

Small workaround

If the message is urgent, try copying its content into a new email or marking it unread before moving it, since some users report that the move then succeeds.

TL;DR: Uncheck the Conversation Clean Up option first, restart Outlook, then update Office if the warning keeps coming back.