This problem cropped up in the last couple days. I download from my Gmail account via IMAP. Occasionally, when I open an email message in Mail, completely the wrong email shows up. Instead of the message I just received, I see some random message from weeks or months ago. I have no idea what might have caused this, and I haven't discerned a pattern as to which messages show this error, or which wrong email shows up. I have to go back to the web version to read the correct message. Does anyone have any ideas as to the cause, or the fix?