My Apple Mail is giving me disconcerting pop-up messages.

Hi Ben,

While we can discern that Mail seems to be attempting to move a message to another mailbox, we have no way of knowing which mailbox or message it's referring to.

If you aren't already, we'd recommend updating to the latest version of macOS available for your device:

"

Before continuing, it's a good idea to back up your Mac.

From the Apple menu  in the corner of your screen, choose System Preferences.

Click Software Update in the System Preferences window. Can't find Software Update?

Software Update now checks for new software.

• If Software Update finds new software, click Update Now or Upgrade Now to install. You might be asked to restart or enter an administrator password.
• If Software Update says that your Mac is up to date, then no new software is currently available for your Mac model.

"

Update macOS on Mac - Apple Support

If you are, or still experience this, we'd recommend reaching out to Apple to look into this with you further: Apple Support

Cheers!