Mail Tool is confused about the state of your Mail File.

Cause

This message appears in a pop-up dialog box whenever you ask mailtool(1) to access messages after another mail reader has modified your inbox. A request follows: Please Quit this Mail Tool.

Action

Click continue to close the dialog box, then exit mailtool(1). If you continue trying to read mail, messages deleted by the other mail reader will never appear, and mailtool(1) will fail to see any new messages.